Empty group blowout fix.

The last element of $group was returning a bool(false) element and throwing an exception:

```Notice: Undefined offset: 0 in vendor/ztec/security-active_directory/Ztec/Security/ActiveDirectoryBundle/Security/User/adUserProvider.php line 113
500 Internal Server Error - ErrorException```

This occurred on on all 2008R2 machines I had access to test on and across several different domains.  Added a quick check to see if the Boolean is false.