done fix to ensure right return type

Test Code

/** @var kDBItem $object */
$object = $event->getObject();
$object->SetError('CouponId', 'required');

Test

1. Login to adm.console
2. Go to the E-commerce → Discounts & Certificates section, go to the Coupons tab
3. Create coupon record with records on Items tab (enough to have one record, produced with "Entire Order" button)
4. Edit coupons_event_handler.php - add test code in the OnBeforeItemCreate, after parent method call.
5. Go to the E-commerce → Discounts & Certificates section, go to the Coupons tab
6. Select some coupon record and press "Clone" toolbar button
7. Go to the Logs & Reports → System Log section
8. Confirm that in the system log is not present notice like this: "Undefined index: coup.-item";
9. Remove test code from coupons_event_handler.php