Fix a warning: PHP Deprecated: mb_strtolower(): Passing null to parameter #1 ($string) of type string is deprecated

Problem/Motivation

When running phpunit tests with the coverage report, it produces a deprecation warning on fresh PHP:

Testing /builds/project/test_helpers/web/modules/custom/test_helpers
.PHP Deprecated:  mb_strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in /builds/project/test_helpers/web/core/lib/Drupal/Core/Config/Entity/Query/Condition.php on line 39

The issue is actually in the Drupal Core, but probably we can invent a workaround for it in the module.