Empty ICS when event is not public

When we register to a private event, the receipt contains an empty ics file.

We should do one of the following :

• allow ics for private event :

-    $info = CRM_Event_BAO_Event::getCompleteInfo($start, $type, $event_id, $end);
+    $info = CRM_Event_BAO_Event::getCompleteInfo($start, $type, $event_id, $end, FALSE);

• don't insert the ics if $info is empty

@bgm I can create a PR but what do you think would be best ?