Oblitus submitted a new mod: [SMAPI] Empty Hands - A mod that allows you to select your bare hands. Read more about this mod...
Correct me if I'm wrong, but I believe this is already a feature in vanilla. Unless I've had this mod before I even started modding Starbound. Please correct me if I'm not remembering right though.
Vanilla allows you to select an empty slot, but you must have one, and they usually are on the right side, so you have to either use a mouse or move your hand from WASD.