convert null values into empty string in laravel 5.4

I want to convert all null with empty string I have used array_walk_recursive but I haven't got what I want please help me to figure it out what I have done wrong here.

protected function setData($key, $value)
{
    $this->data[$key] = $value;
    array_walk_recursive($this->data, function (&$item, $key) {
        $item = null === $item ? '' : $item;
    });
    return $this->data;
}