I looked on the Harry Potter wikia, and it said it was a blank diary but I don't remember that being mentioned in the books.
It seems like yes, Tom Riddle had written in the diary.
Dumbledore tells Harry that the reason Tom made the diary a Horcrux was because it held proof that he was the heir of Slytherin.
“The diary, as you have said yourself, was proof that he was the heir of Slytherin; I am sure that Voldemort considered it of stupendous importance.” - Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
Since this would have been before it was a Horcrux, it being a Horcrux obviously wouldn't have had anything to do with how it had proof that he was the heir of Slytherin. Therefore, the most likely way for it to have this proof before being a Horcrux is for Tom to have written something that proves it in the diary.