Allow PhutilClassMapQuery to filter null

Summary: Allow PhutilClassMapQuery to filter objects which return null from the "unique" method.

Test Plan: See next diff.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13572