refactor: add |null in docblock

[alipowerful7]

add return null in this function

    /**
     * Get the date format for an attribute if it has one.
     *
     * @param  string  $attribute
     * @return string|null
     */
    protected function getDateFormat($attribute)
    {
        if ($result = $this->getRule($attribute, 'DateFormat')) {
            return $result[1][0];
        }

        return null;
    }

and add |null in this docblock

    /**
     * Get the date timestamp.
     *
     * @param  mixed  $value
     * @return int|null
     */
    protected function getDateTimestamp($value)
    {
        $date = is_null($value) ? null : $this->getDateTime($value);

        return $date?->getTimestamp();
    }