R1:76cd181bf379

Don't try to emit project board update events if there are no projects to update

Summary: Ref T4900. We may execute a bad query here if the task has no projects at all.

Test Plan: Edited a task with no new or old projects. Instead of an exception, things worked.

Maniphest Tasks: T4900

Differential Revision: https://secure.phabricator.com/D20689