I would say that this question probably can't be fully answered until the movie releases. (As some have already mentioned in the comments, the answer to this question will probably be a rather important plot-point in the movie itself.)
However, it is possible that the Falcon was originally equipped with a small shuttlecraft that docks on the front of the main vessel, and this is what we're seeing as that extra place that is filled in on the nose of the ship.
The reason I say this is that we know other Correllian ships are made with this "accessory," such as the VCX-100 light freighter, like the Ghost from Star Wars Rebels. This freighter is equipped with the exact same type of mini-ship (the one on the Ghost is known as the Phantom), though it docks on the back of that ship.
Here's a picture of the Ghost, with Phantom intact:
Here's one of the Phantom itself:
And here's a shot of the ship as seen in Rogue One without the Phantom:
(apologies for the quality of this last one, it was the only shot of the Phantom-less Ghost that I could find)
[I hope this answer isn't based too much on simple conjecture or fan-speculation, and must thereby be removed; I honestly think this particular question, while very interesting, is more or less impossible to answer until Solo: A Star Wars Story officially releases unless some spoilery article somehow gets leaked before then.]