Fix getItemProjectsPHIDs null exception

Summary:
Phabricator API can sometimes return an empty array.
In such case, we don't have any projects to return.

This fix the following issue:
Trying to get property of non-object in
PhabricatorStory::getItemProjectsPHIDs method

Fixes T916.

Test Plan: Fire again the failed notification.

Reviewers: dereckson

Maniphest Tasks: T916

Differential Revision: https://devcentral.nasqueron.org/D488