[D7 PHP 8.1] check_plain(): Passing null to parameter #1 check_plain() includes/bootstrap.inc, line 1907

Problem/Motivation

Follow-up from #3224299-40: [META] Make Drupal 7 core compatible with PHP 8.1

I'm seeing this on ctools ATM, in the midst of tracking it down but thought I'd ask

exception: [Deprecated] Line 1898 of includes/bootstrap.inc:
htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated

EDIT: to give more context to why I ask, is that I think happening very generically in ctools, for example:

  protected function assertPluginMissingFunction($module, $type, $id, $function = 'function') {
    $func = ctools_plugin_load_function($module, $type, $id, $function);
    $this->assertEqual($func, NULL, t('Plugin @plugin of plugin type @module:@type for @function with missing function successfully failed.', array(
      '@plugin' => $id,
      '@module' => $module,
      '@type' => $type,
      '@function' =>$func,
    )));

Where one of those @ variables in t() is being passed to check_plain as either FALSE or NULL (null in this case). There are probably more places than can be counted. I could probably cast them in the test to make it pass (because I think this is testing missing plugins)

Steps to reproduce

Proposed resolution

Remaining tasks

Write tests

User interface changes

API changes

Data model changes

Release notes snippet