Backport #25278 by @Zettat123
Fix#21072
Username Attribute is not a required item when creating an
authentication source. If Username Attribute is empty, the username
value of LDAP user cannot be read, so all users from LDAP will be marked
as inactive by mistake when synchronizing external users.
This PR improves the sync logic, if username is empty, the email address
will be used to find user.
Co-authored-by: Zettat123 <zettat123@gmail.com>
dfefe86045