Update how "on_use" is defined for potions.

mods/ITEMS/mcl_potions/potions.lua

@@ -54,13 +54,19 @@ local function register_potion(def)
 		dur = 45
 	end
 
-	local on_use = function (itemstack, user, pointed_thing)
-							if not def.on_use then return end
+	local on_use = nil
+
+	if def.on_use then
+
+		on_use = function (itemstack, user, pointed_thing)
+
 						def.on_use(user, def.effect, dur)
 						minetest.do_item_eat(0, "mcl_potions:glass_bottle", itemstack, user, pointed_thing)
 						mcl_potions._use_potion(itemstack, user, def.color)
+
 						return itemstack
 						end
+	end
 
 	local function get_tt(tt, effect, dur)
 		local _tt