Fix an issue with editing application default policies in Calendar

Summary: Ref T11816. We're running this code on empty events which haven't been initialized and don't have a source attached -- just use a more explanatory check which doesn't need anything attached.

Test Plan: Edited default Calendar policies.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D16803