pager_test_preprocess_pager() should return if pager element is not defined

Problem/Motivation

#1202484: Improve usage of 'pager.manager' service in PagerSelectExtender, allow code to know the pager element ID used is causing a negative side effect in contrib, see #3201324: Tests fail on Drupal 9.2.

If a pager is created with a gap in the sequence of the pager elements, pager_test_preprocess_pager() fails when trying to preprocess not exixting pagers.

The template processor in theme.inc, template_preprocess_pager(), has protection for this

  // Nothing to do if there is no pager.
  if (!isset($pager)) {
    return;
  }

Proposed resolution

Copy the protection code to pager_test_preprocess_pager().

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet