Protect on_pick_up against unsafe usage

mods/ENTITIES/mcl_mobs/api.lua

@@ -3134,9 +3134,9 @@ local function check_item_pickup(self)
 				for k,v in pairs(self.pick_up) do
 					if not player_near(p) and self.on_pick_up and l.itemstring:find(v) then
 						local r =  self.on_pick_up(self,l)
-						if  r and r:get_count() > 0 then
+						if  r and r.is_empty and not r:is_empty() then
 							l.itemstring = r:to_string()
-						else
+						elseif r and r.is_empty and r:is_empty() then
 							o:remove()
 						end
 					end