[LDAP] Login: dont fail with 500 if configured display name attribute is not set

If a user has no display name, login will end up with a server error. Instead now, login is still not possible, but ownCloud does not result in a blank page and a corresponding log messages is written. Unit test included. Fixes #11762 

How to test?
- configure LDAP
- set display name attribute to something not existing
- attempt to log in with an LDAP user
- before the fix: blank page
- now: login mask again, log message: 'LDAP Login: Could not get user object for DN  $dn  Maybe the LDAP entry has no set display name attribute?'

Please test and review @theonlydoo @Xenopathic @PVince81 @craigpg