Don't notify without notifiable attendees

Summary: Events with no attendees (e.g. fresh instances of recurring events) would trigger an exception when sending notifications, because $attendee_map would still get populated.

Test Plan: Declined event, restarted daemons. Did not see exception. Accepted event, restarted daemons. Saw "[Calendar] [Reminder]" email.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D18835