Be more liberal in reading LDAP configuration
Summary: Ref T1536. If we have '' (empty string) instead of a strict null, we should still ignore it during lookup.
Test Plan: Configuration with searchAttibute but empty string usernameAttribute now works correctly.
Reviewers: btrahan, chad
Reviewed By: chad
CC: aran
Maniphest Tasks: T1536
Differential Revision: https://secure.phabricator.com/D6286