Fix system notice in manage_user_page.php
Fix My Account page can't change the real name

Regression caused by a80f447687722b52160d00c96c21a5be5f4329e1 which introduced
strict comparison on return value of user_get_id_by_name. The function
returns false when user id is not found, but the code was still
comparing against 0.

Fixes #16519
Fix #16515: Manage custom field page crashes