Fix an issue where setting a recurrence end date on a Calendar event without one could fatal

Summary: Ref T11816. The underlying format of recurrence end dates swapped around a bit and we now try to compare null to a valid date if you're setting it for the first time.

Test Plan:

• On a new event, set a recurrence end date.
• Then, removed a recurrence end date.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D17150