R1:dc558b5538cd

Fix NULL pointer exception in some circumstances from Calendar's homepage

Summary:
After importing specific weird events, for example from Google Calendar (bleah), it can happen that the Calendar's homepage becomes broken.

This was the Exception error shown to video:

"Call to a member function getEventsBetween() on null"

It was happening since this method can return NULL:

PhabricatorCalendarEventQuery#newRecurrenceSet()

This changeset verifies this condition from the Calendar's homepage.