fix: [#3489656] Set $styles to empty array if no third party settings exist

See https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Config%21Entity%21ConfigEntityBase.php/function/ConfigEntityBase%3A%3AgetThirdPartySetting/8.9.x

This ensures the array_filter call on line 671 !41 (diffs) gets an empty array instead of null.

Closes #3489656