Remove end_time requirement

As it's not compulsory to have an end time on events is would be good if there was a check in place to see if the field is empty so an error isn't thrown.

I've found changing FBAutopostEntityEvent.php to

 if (!empty($publication['params']['end_time']) && is_numeric($publication['params']['end_time'])) {
      $end = new DateTime('@' . $publication['params']['end_time']);
      $publication['params']['end_time'] = $end->format(DateTime::ISO8601);
 }

resolves this issue.