[8.x] Fix php 8.1 deprecation in ValidatesAttributes::checkDateTimeOrder

[bastien-phi]

Calling DateTime::createFromFormat($format, null) is deprecated with PHP 8.1

Validating

Validator::make(
    ['start' => '31/12/2012', 'ends' => null],
    ['start' => 'date_format:d/m/Y|before:ends', 'ends' => 'date_format:d/m/Y|after:start']
)

triggers this deprecation in ValidatesAttributes::getDateTimeWithOptionalFormat.

[bastien-phi]

Thanks !