Maniphest - fix fatal in custom query for users with no projects
Summary: See https://github.com/facebook/phabricator/issues/380 for report.
Test Plan: Maniphest -> Custom Query -> Put user with no project in "Any User Projects" field -> Search : observe no fatal
Reviewers: epriestley
Reviewed By: epriestley
CC: Korvin, aran
Differential Revision: https://secure.phabricator.com/D6862