Fix null exception when Phabricator repository is private

Summary:
When a repository is private and can't be reached by the bot account
used to fetch Phabricator information, a null exception occurs in
PhabricatorStory::getRepositoryPHID method.

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

Fixes T1136.

Test Plan: Fire again the notification.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1136

Differential Revision: https://devcentral.nasqueron.org/D872