refactor: check for empty/false coupons in order

src/QUI/ERP/Coupons/Events.php

@@ -107,6 +107,10 @@ public static function onQuiqqerOrderSuccessful($Order)
     {
         $coupons = $Order->getDataEntry('quiqqer-coupons');
 
+        if (empty($coupons)) {
+            return;
+        }
+
         foreach ($coupons as $couponCode) {
             try {
                 $CouponCode = CouponsHandler::getCouponCodeByCode($couponCode);