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.

Closes T15136

Test Plan: I tried in my server. I've executed the syntax lint. On my local machine I was not able to run "arc diff" since it tries to connect to root@localhost for some reasons.

Reviewers: O1 Blessed Committers, 20after4

Reviewed By: O1 Blessed Committers, 20after4

Subscribers: 0, Cigaryno, 20after4, speck, tobiaswiese, Matthew

Tags: #calendar

Maniphest Tasks: T15136

Differential Revision: https://we.phorge.it/D25060