Fixed request parameter for error presenter to be either instance od Nette\Application\Request or NULL

I've tried to add typehints to ErrorPresenter:
```php
final class ErrorPresenter extends BasePresenter
{
	public function renderDefault(\Exception $exception, \Nette\Application\Request $request = NULL)
	{
		// ...
	}
}
```
It didn't work as expected because when there was no request the end function returned FALSE and not NULL.