Do not rename the anonymous user in hook_user_presave()

Saving user 0 while email_registration is installed renamed it to "user", because the presave hook generated a username from the empty email address via email_registration_cleanup_username(). The anonymous account must keep an empty name so that getDisplayName() falls back to the user.settings:anonymous label.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com