fix: returning air when item was empty

Author: SuperCrafting

src/main/java/pt/supercrafting/menu/slot/MenuSlot.java

@@ -127,7 +127,7 @@ default void add(Add add) {
             ItemStack newItemStack = itemStack.asQuantity(amount);
             itemStack(newItemStack);
 
-            ItemStack result = ItemStack.empty();
+            ItemStack result = itemStack.asQuantity(itemStack.getAmount() - amount);
             add.setResult(result);
             return;
         }