Fix fatal on project boards with no tasks

Summary: Ref T1344. We try to do a bad edge query with no sources right now if there are no tasks in a project.

Test Plan:
  - Hit exception, applied patch, no exception.
  - Other boards still have tasks.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1344

Differential Revision: https://secure.phabricator.com/D7951