[5.0] Fix broken file upload when submitting a form without selecting a file

This is my attempt at fixing (my first PR) the issue:
[5.0] File upload is broken when submitting a form without selecting a file #6189

Right now I'm overriding Symfony's Request::duplicate function so that it can handle empty file form inputs. Another solution is included in the issue (#6189), which modifies the createFromBase function. Both filter the input array to remove any files that are null.
Why not just ?
```php
/**
 * {@inheritdoc}
 */
public function duplicate(array $query = null, array $request = null, array $attributes = null, array $cookies = null, array $files = null, array $server = null)
{
	$files = array_filter((array) $files);

	return parent::duplicate($query, $request, $attributes, $cookies, $files, $server);
}
```