[5.3] Deal with null $files in request

[themsaid]

In Request::duplicate:

public function duplicate(array $query = null, array $request = null, array $attributes = null, array $cookies = null, array $files = null, array $server = null)
{
    return parent::duplicate($query, $request, $attributes, $cookies, $this->filterFiles($files), $server);
}

Since files can be null we should skip the loop in that case to avoid Invalid argument supplied for foreach()