Can't do it directly, they overlooked that for some reason - only chance is to make a weapon called "Bare Hands" or something and force it to be equipped at all times by checking your character's equipment. ...Naturally, you can check if "Sword" or "Sword 2" is equipped, but not check if you have 'nothing' equipped in that slot (thanks Enterbrain) so you'll have to do a long series of branches that will look something like

And you'll have to nest every available weapon for that character, and in the center, most nested area - have it equip "Bare Hands". If the person removes "Bare Hands" (say, via equipping "Awesome Sword") then just have the game discard all "Bare Hands" items.

I'd go into more detail but I have somewhere to be in a little while