Account for null when no more cart items are present

shop.sql

@@ -121,10 +121,11 @@ after delete
 on cart_item for each row
 begin
 	update cart
-	set price_total = (
-		select sum(price_subtotal)
+	set price_total = ifnull(
+		(select sum(price_subtotal)
 		from cart_item
-		where cart_id = old.cart_id
+		where cart_id = old.cart_id),
+		0.00
 	)
 	where id = old.cart_id;
 end;