Fixing strictly null checks in CalendarInviteeQuery
Summary: Ref T7935, Fixing strictly null checks in CalendarInviteeQuery
Test Plan: Check code.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T7935
Differential Revision: https://secure.phabricator.com/D12615