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