Query for recurring event parents should accept a "not a child of anything" clause

Summary: Closes T8371, Query for recurring event parents should accept a "not a child of anything" clause

Test Plan: querying for a list of events with recurring events should still work the same.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8371

Differential Revision: https://secure.phabricator.com/D13140