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.
- Hit exception, applied patch, no exception.
- Other boards still have tasks.
Reviewers: btrahan, chad
Reviewed By: chad
Maniphest Tasks: T1344
Differential Revision: https://secure.phabricator.com/D7951