Check if recurrence end date is disabled before saving it.

Summary: Ref T8357, Check if recurrence end date is disabled before saving it.

Test Plan: Create new event, before saving, leave "recurrence end date" unchecked, save, should not get an error.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T8357

Differential Revision: https://secure.phabricator.com/D13212