Run on_item_use CSM callback even if item is not marked usable

Mods may want to to handle item interaction even if the item
is not marked usable (= server-side callback exists).

clientmods/preview/init.lua

@@ -66,6 +66,10 @@ core.register_on_item_use(function(itemstack, pointed_thing)
 	print("pointed_thing :" .. dump(pointed_thing))
 	print("item = " .. itemstack:get_name())
 
+	if not itemstack:is_empty() then
+		return false
+	end
+
 	local pos = vector.add(core.localplayer:get_pos(), core.camera:get_offset())
 	local pos2 = vector.add(pos, vector.multiply(core.camera:get_look_dir(), 100))