PHP deprecation triggered by OpenIDConnectSessionTest

Problem/Motivation

A PHP deprecation notice is triggered by OpenIdConnectSessionTest::testSaveDestinationUserPath():

1 test triggered 1 PHP deprecation:

1) /var/www/html/modules/contrib/openid_connect/src/OpenIDConnectSession.php:102
ltrim(): Passing null to parameter #1 ($string) of type string is deprecated

Triggered by:

* Drupal\Tests\openid_connect\Unit\OpenIdConnectSessionTest::testSaveDestinationUserPath
  /var/www/html/modules/contrib/openid_connect/tests/src/Unit/OpenIdConnectSessionTest.php:147

Eventually this will cause an error when the module is tested on PHP 9. It's caused by OpenIDConnectSession::saveDestination() trying to build a URL for the user.login route.

Steps to reproduce

See the most recent max PHP version pipeline: https://git.drupalcode.org/project/openid_connect/-/jobs/3855616.

Proposed resolution

The OpenIdConnectSessionTest::testSaveDestination() test gets around this by mocking the route. A quick fix is to do the same mocking in testSaveDestinationUserPath(). But a more appropriate fix might be to check for the existence of the route in saveDestination() or to simply add the User module as a dependency. I'll submit an MR for the quick fix and leave it up to the maintainers to decide whether they want to bother making the code more robust.