T5423, "is newly created" herald rule fails on dry runs

Summary: Fixes T5423, "is newly created" herald rule fails on dry runs

Test Plan: Create herald "is newly created" rule, and do a dry run on an existing pholio mock, differential commit, or maniphest task. Should not return an exception.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5423

Differential Revision: https://secure.phabricator.com/D10187