Application Emails - conditionally pass around the application email

Summary: due to typehints, passing null is going to barf here. Ref D11564, ref T5039.

Test Plan: made an edit to a task from the web ui and it didnt fatal

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5039

Differential Revision: https://secure.phabricator.com/D11571