Event detail view of Calendar events with no invitees should show "None" in the Properties column.

Summary: Ref T8020, Event detail view of Calendar events with no invitees should show "None" in the Properties column.

Test Plan: Open event, edit event to have no invitees, save, event detail view should show "None" in the Invitee section of the Properties column

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8020

Differential Revision: https://secure.phabricator.com/D12718