Support for null returns

I would be useful for the models to have null return types where needed as otherwise we need to add phpstan overrides in our code. An example being Google\Service\Calendar\Event->getOrganizer() which says it always returns an EventOrganizer when it can also return null.

Describe the solution you'd like

For the return to be marked as null e.g.

  /**
   * @return EventOrganizer|null
   */
  public function getOrganizer()
  {
    return $this->organizer;
  }