Access forbidden for systempay init form

Problem/Motivation

The systempay.systempay_init_form route return AccessResult::forbidden.
This because \Drupal::currentUser()->id() return a string or an int.

Proposed resolution

Force convert to integer, replace:

if ((int) $payment->getOwnerId() !== $this->currentUser->id()) {

by:

if ((int) $payment->getOwnerId() !== (int) $this->currentUser->id()) {