Only evaluate tokens if phids not null

Summary: Fixes T8577, Only evaluate tokens if phids not null

Test Plan: Open Upcoming Events in Calendar.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8577

Differential Revision: https://secure.phabricator.com/D13326