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

src/applications/calendar/query/PhabricatorCalendarEventQuery.php

@@ -222,12 +222,16 @@ final class PhabricatorCalendarEventQuery
 
       $limit = $this->getRecurrenceLimit($event, $raw_limit);
 
+      // note that this can be NULL for some imported events
       $set = $event->newRecurrenceSet();
 
-      $recurrences = $set->getEventsBetween(
-        $start_date,
-        $end_date,
-        $limit + 1);
+      $recurrences = array();
+      if ($set) {
+          $recurrences = $set->getEventsBetween(
+            $start_date,
+            $end_date,
+            $limit + 1);
+      }
 
       // We're generating events from the beginning and then filtering them
       // here (instead of only generating events starting at the start date)