If you mean just opening the project for the first time just to not wait for unity to load during the actual game jam time, that is allowed. But if you mean work on the game for the competition before it starts, that is not allowed and you would also not know the theme.
Well what I mean by "Build" is to compile the project into an executable game.
The first time you do this in Unity, it can take ages, even if your project is completely untouched from the time it was ever opened. So my question is can I compile the empty project right now, so that when I actually create and finish the game, it wont take so long to compile into an executable.