Fix a possible query error in filtering events

If all events are filtered by range logic, this invitee query may be invalid.

Auditors: lpriestley

src/applications/calendar/query/PhabricatorCalendarEventQuery.php

@@ -266,11 +266,15 @@ final class PhabricatorCalendarEventQuery
       $phids[] = $event->getPHID();
     }
 
+    if ($events) {
       $invitees = id(new PhabricatorCalendarEventInviteeQuery())
         ->setViewer($this->getViewer())
         ->withEventPHIDs($phids)
         ->execute();
       $invitees = mgroup($invitees, 'getEventPHID');
+    } else {
+      $invitees = array();
+    }
 
     foreach ($events as $event) {
       $event_invitees = idx($invitees, $event->getPHID(), array());