Cast items to array.

Fix issues when items are null and php raises "Invalid argument supplied for foreach()".

src/LaraCart.php

@@ -747,7 +747,7 @@ public function totalDiscount($format = true, $withItemDiscounts = true)
 
         if ($withItemDiscounts) {
             /** @var CartItem $item */
-            foreach ($this->cart->items as $item) {
+            foreach ((array) $this->cart->items as $item) {
                 $total += floatval($item->getDiscount(false));
             }
         }