Part One of Three. A sort of an answer.
Micah in comments says the Terran Empire is ruled by a "faceless Emperor". If the monarch's identity is kept secret (to avoid assassination attempts?), everyone assumes that "the present ruler of the empire" is someone whose age, gender, personality, hobbies, education, and other personal details they don't need to know and are discouraged from knowing. Since trying to discover personal details about the monarch might be considered preparation for an assassination or coup attempt.
Unless it is known that there have been at least 50 percent female monarchs, it would be natural to assume that the monarch is male, and also that it doesn't matter whether the hidden emperor is male or female.
So I guess it is usual to refer to "the emperor" instead of to "the emperor or empress as the case may be". And people might say that the emperor is equally the emperor whether male or a female since they aren't going to know anyway.
Part Two. List of Titles of Terran Empire rulers.
2155: Male emperor, female empress.
2255: female emperor.
Sometime between 2267 and 2370: Commander-in-chief?
in TOS "Mirror, Mirror" (October 6, 1967) with an official fictional date of 2267,
MARLENA: You're still in trouble with Starfleet Command. What you've got in mind this time is beyond me. You're scheming, of course. The Halkans have something you want, or, is it all some clever means to advance you to the Admiralty? Kirk. The Cabinet itself?
KIRK: Further than that, if I'm successful.
MARLENA: Really? Well, you must know what you're doing. You always do. If I'm to be the woman of a Caesar, can't I know what you're up to?
In the DS9 episode "Crossover" (May 15, 1994) with a official fictional date of 2370 Kira is told the history of the Klingon-Cardassian Alliance:
INTENDANT: Perhaps you'd recognise the name Kirk.
KIRA: I'm sorry, I don't.
INTENDANT: Interesting. On my side, Kirk is one the most famous names in our history. Almost a century ago, a Terran starship Captain named James Kirk accidentally exchanged places with his counterpart from your side due to a transporter accident. Our Terrans were barbarians then, but their Empire was strong. While your Kirk was on this side, he met a Vulcan named Spock and somehow had a profound influence on him. Afterwards, Spock rose to Commander in Chief of the Empire by preaching reforms, disarmament, peace. It was quite a remarkable turnabout for his people. Unfortunately for them, when Spock had completed all these reforms, his empire was no longer in any position to defend itself against us.
INTENDANT: The Alliance. The historic coming together of the Klingons and the Cardassians.
Here it is said Spock became "commander-in-chief" of the Terran Empire. Since the Terran Empire was overthrown decades before 2370, Mirror Universe stories with fictional dates after 2370 probably give a lot more information about the politics of the Klingon-Cardassian Alliance than about the politics of the long gone Terran Empire.
But there is another episode set before the fall of the Terran Empire: the Enterprise episode "In a Mirror, Darkly" (April 22 & 29, 2005), with an official fictional date of 2155.
FORREST: ...The Emperor pins a medal on your chest...
HOSHI: ...Can you imagine the look on the Emperor's face when he sees this ship?...
ARCHER: (his subconscious again) ...He's going to present this ship to the Emperor himself...
TPOL: ...Archer intends to overthrow Starfleet and the Emperor.
TPOL: ...If he becomes Emperor, he'll lay waste to Vulcan...
HOSHI: I've never been the consort of an Emperor before...
TPOL: Archer plans to kill the Emperor and take his place.
TPOL: ...If someone saves the Emperor's life, it's said there's no request he can refuse them...
PHLOX: If the Emperor were my patient, I'd be obligated to save his life...
ARCHER: They're loyal to the Emperor. They don't really care who it is. By the time the fleet arrives, the reign of Emperor Jonathan Archer will have already begun.
HOSHI: You're speaking with Empress Sato. Prepare to receive instructions.
So in 2155 a male monarch of the Terran Empire is an emperor and a female trying to usurp the throne calls herself an empress.
Part Three: Historical female rulers with masculine or unisex titles.
A number of ancient Egyptian queens became regents for minor kings. And some of them may have become monarchs in their own right. Hatshepsut (died 1458 BC) was the most famous female pharoah or king of ancient egypt.
in the early Roman empire, the imperial titles were Imperator, Caesar, Augustus, and Princeps. The highest title granted to a woman of the Imperial family was Augusta, the female form of Augustus. The title Augusta was granted by the Emperor and the Senate to specific women - there were times when the Emperor's wife was not an Augusta, and there were times when two or more women had the title of Augusta.
In 263 Odaenathus, ruler of Palmyra and Corrector totis Orientis ("Righter of The Entire East") became King of Kings of the East and crowed his oldest son Hairan I as co-King of Kings. In late 267 Odaenathus and Hairan were assassinated and Vaballathus (c. 259- after 274) became King of Kings. Queen of Kings Zenobia ruled as regent in the name of Vaballathus.
Palmyrene forces conquered Egypt and Asia Minor in 270-271. Vaballathus was proclaimed Augustus (emperor) and Zenobia, still his regent, was proclaimed Augusta. Thus Zenobia was close to being a female emperor, but presumably would soon have to give more and more power to her son as he grew older. Emperor Aurelian soon crushed their rebellion.
Ulpia Severina, wife of Emperor Lucius Domitianus Aurelianus Augustus (Aurelian), was granted the title of Augusta in 274, and also had the titles of Pia ("pious") and Mater castorum et Senatus et Patriae ("Mother of the camps, the senate, and the country").
Aurelian was assassinated in September or October 275, and Marcus Claudius Tacitus Augustus was elected emperor by the Senate. There is some evidence that Ulpia Severina may have ruled during the interregnum. Some of her coins seem to have been minted after Aurelian's death - I don't know if her title on those coins was Augusta or Augustus.
The Chinese emperors used the title of Huangdi. The highest wife of an emperor was a Hou, which is not very close to Huangdi. A mother of a reigning emperor was often given a senior title like Tai Hou. A number of women ruled as regents for minor emperors or were the powers behind the throne.
During the Tang Dynasty Wu Zetian (died 705) ruled in the name of various relatives for 30 years from 660 to 690 and then proclaimed her own dynasty, the Zhou, and became the only female Huangdi.
In the 7th century the title of the Roman Emperor ruling in the East became the Greek Basileus. The wives or other female relatives of the emperors were often given the title of Basilissa. Irene (c. 752-803), widow of Emperor Leo IV, ruled as regent for her son Constantine V from 780 to 790. In 797 Irene deposed Constantine V and ruled in her own right until deposed in 802.
Many historians claim that Irene used the title of Basileus instead of Basilissa, but actually all her documents and coins used Basilissa except for 2 documents and one coin.
Charlemagne was crowned emperor in Rome in 800 AD, beginning a line of Holy Roman Emperors in western Europe using the Latin title of Imperator Augustus. Theophanu (c. 955-990), a Byzantine lady with highly speculative ancestry, married the co-emperor OttO II and was crowned empress in 972. She was regent for their son Otto III from 983-990, issuing diplomas as Imperator Augustus, Emperor.
When Emperor Constantine VIII died in 1028 his daughter Zoe (c. 978-1050) reigned as empress, usually leaving the ruling to her three successive husbands who became emperors. Her sister Theodora (980-1056) was forced to become junior co empress in 1042 but ruled for a few months in Zoe's name until Zoe married her third husband. Theodora retired in 1050 but became monarch again in 1055 and ruled alone until her death. She may have used the title Basileus instead of Basilissa.
Jadwiga (1373/74-1399) became King of Poland in 1384, and her sister Mary (1371-1395) was King of Hungary in 1382-1385 and 1386-1395. Anna Jagiellon (1523-1596) became co-king of Poland with her husband from 1575-1587. Empress Maria Theresa (1717-1780) was King of Hungary from 1740 to 1780.