Fixed broken handle_schedule_reply

When `handle_schedule_reply` is called with a vCALENDAR missing an organizer, the code crashes:

```php
  $organizer = $ical->GetOrganizer();
  // for now we treat events with out organizers as an error
  if ( count( $organizer ) < 1 ) return false;
```

due to the fact that `$ical->GetOrganizer()` returns 'FALSE', if no organizer is given:

```php
function GetOrganizer() {
    if ( !isset($this->organizer) ) {
      $organizers = $this->GetPropertiesByPath('/VCALENDAR/*/ORGANIZER');
      $organizer = (count($organizers) > 0 ? $organizers[0] : false);
      $this->organizer = (empty($organizer) ? false : $organizer );
      ---snip---
    }
    return $this->organizer;
  }
```

I changed the check to use 'empty' instead which takes 'FALSE' and empty arrays.