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