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;
}
```