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