Unable to edit user profiles where "username" field is disabled

When editing a user profile which has the "username" field disabled, such as for a user who's authentication method is set to "OAuth 2", the form validation fails with a reason that "username is required".

Steps:

1. Search for a user account which has an authentication method that disables the editing of a username.
2. Click on the "edit" action.
3. Tick the "Suspended account" checkbox.
4. Click the "Update profile" button.