Fix null exception when Phabricator object is private

Summary:
When an object like a paste is private and can't be reached by the bot
account used to fetch Phabricator information, a null exception occurs
in PhabricatorStory::getItemProjectsPHIDsThroughApplicationSearch().

This change handles the case an API query doesn't contain the
required information.

Fixes T1138.

Test Plan: Fire again the notification.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1138

Differential Revision: https://devcentral.nasqueron.org/D873