fix checking usergroups

$oPayPalPayment->getGroups() always returns an oxlist object, which might have no oxGroup items but even an empty oxList is never empty(oxList).
This is already fixed in 3.2.1:
https://github.com/OXID-eSales/paypal/blob/b-3.2/source/modules/oe/oepaypal/models/oepaypalpaymentvalidator.php#L292