Return anonymous user as event owner if it has been deleted

The current implementation returns NULL. This is not an allowed return value based on the Drupal Core EntityOwnerInterface.

The agreed upon approach to addressing this situation seems to be to return the anonymous user as the owner if the user does not exist. Update the code accordingly.

The implementation is inspired by https://www.drupal.org/project/drupal/issues/3335025

Closes #3515976