Fix an issue where 'Attending' would appear on calendar view unnecessarily

Summary:
Ref T10295

• Viewing Upcoming Events in the calendar would display 'Attending: ' even if there were not attendees. This caused confusion, such as 'Is it telling me I am "Attending?"'
• When a calendar event has no attendees, simply do not display the 'Attending: ' label

Test Plan:

• Add a new event with no one attending.
• Add a new event with one or more attendees.
• View the Upcoming Events query of the Calendar app.
• Notice how the one with no attendees does not show 'Attending: ' while the other with attendees will show the already existing 'Attending: jdoe, ssmith' label.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T10295

Differential Revision: https://secure.phabricator.com/D15207