[events/google/parsing] Handle cancelled event parsing more gracefully.

Summary:
Quick fix for parsing google calendar events that have been cancelled.

Note that a better solution will be put in shortly to determine if we've synced an event that has since been cancelled and take care of it accordingly.

Test Plan: Run tests which now exercise cancelled events and make sure they pass.

Reviewers: emfree

Reviewed By: emfree

Maniphest Tasks: T278

Differential Revision: https://review.inboxapp.com/D313