Support no game cores

[FlorianUekermann]

Supporting no-game cores should be easy. I didn't send a patch, because there are some decisions to be made. I would suggest adding a support_no_game: bool to CoreInfo and either turning the game_data argument in on_load_game( &mut self, game_data: libretro_backend::GameData ) into an option, or simply setting its fields to None.

[koute]

Since GameData is already designed as a container which can be empty I think it would be more natural to just set the fields to None, and adding fn supports_no_game( mut self ) -> Self to CoreInfo sounds fine to me.