You can always technically start a game jam if there's no theme being held back... but it's against the spirit of the thing, and not how it's intended to be done. Kind of takes the fun out of it.
What you can and probably should do -- at least if I'm understanding right, and this is the same for a lot of other jams -- is start your planning now. I'm in the middle of defining my game loop, gathering assets, and laying out how I'll organize my UI and transitions. There are also likely to be details to work out specific to your development environment, depending on what you'll be using to write your game, and those are problems you can research without actually starting your project.
No code gets written that'll actually go into the game, but prep is important, particularly when you only get two weeks of actual implementation time.