fixes possible override of uniqueMember by autodetection

* uniqueMember was the default so we did not know whether this setting is
  desired or the initial value
* autodetection of the user-group association attribute runs only when it
  was not set (as far as we knew)
* the default is now empty
* thus LDAPProvider might return this value as well (in exceptional cases)
* if a group base is given (edge case), use this instead of general base
* resolves #12682

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>