Allow date custom field to be blank

Summary:
Currently it's not allowed to be left blank (even with required: false)

Fixes T3343

Test Plan: Use the custom date field.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, chad

Maniphest Tasks: T3343

Differential Revision: https://secure.phabricator.com/D6251