ConfigExistsConstraintValidator should ignore NULL values

Problem/Motivation

Reported at #3317769-139: Drastically improve the linking experience in CKEditor 5

GitLab CI reports 100% success rate but a job failed 🤔 This is due to a bug in the GitLab integration:

    Remaining self deprecation notices (1)
    
      1x: Deprecated NULL placeholder value for key (@name) in: "The '@name'
    config does not exist.". This will throw a PHP error in drupal:11.0.0. See
    https://www.drupal.org/node/3318826

is not a test failure but triggers a job failure 😅

This is caused by a small bug in ConfigExistsConstraintValidator. Fixed.

Steps to reproduce

Pass in NULL to this validator.

Proposed resolution

Harden this validator like literally every other validator.

Merge request link

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet