Refactoring CalendarEventSearchEngine to fit new null handling of AphrontFormDateControlValue

Summary: Ref T4393, Refactoring CalendarEventSearchEngine to fit new null handling of AphrontFormDateControlValue

Test Plan: Use calendar query for day view, expect no php errors

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T4393

Differential Revision: https://secure.phabricator.com/D12701