empty username should be ordered as last

[i-just]

Description

Ensure that empty usernames are at the end of the sorted results. In other words, if you have usernames: abc, NULL, def and they’re all active, the default sort order should resolve to abc, def, NULL

Bit of history:

• we first adjusted the Users::getUserByUsernameOrEmail() method to prioritise the credentialed users (order by credentialed users first #18154);
• then this code was removed in favour of a more general 856bc2d approach;

Steps to reproduce without involving Commerce:

• login to CP
• create a user with an empty username, and email of test@test.com
  • click “Create and set permissions”; don’t set permissions, don’t send activation email; exit to the user index page
• create another user with username and email both set to test@test.com
  • click “Create and set permissions”
  • set permissions (at least access CP)
  • click “Save and send activation email”
  • activate that account
• logout
• try to log in as test@test.com

Without this PR, you won’t be able to log in, as the first returned user will be the inactive one (one without a username).

With this PR, you should be able to log in as the first returned user will be the active one (one with the username).

Related issues

#18723