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