Add check for empty default ammo type when spawning matching ammo for guns

The ammo type "darts" does not come with a default ammo item type. Because the blowgun is in an item group ("homeguns") which should spawn the matching ammo for guns (`guns_with_ammo:true`), this would spawn an item with an empty string as ident ("darts" has this as default ammo), this shows up as "undefined-s".

This PR adds a check to prevent this behavior. One could additionally define a default ammo item type for darts, but this kind of error might come up for other guns, too.