Handle URLs without query params for PxPay

@odorcicd @bizla /cc @Shopify/payments.

PxPay's token response XML can contain URLs both with and without query params. Because we do PxPay offsite redirect via the use of the ``form method="get"``, whatever query params we receive (if any), should be added as hidden fields. They cannot be preserved as part of the ``form``'s ``action``, because the user agent will strip them out and replace them with ``form``'s fields instead.