Handle nullable for php8.3

[onally01]

Description

PHP 8.3 deprecates implicitly nullable parameters

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[MPJHorner]

I think this will also resolve my error..

PHP Deprecated: CraigPaul\Mail\PostmarkTransport::send(): Implicitly marking parameter $envelope as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/vendor/coconutcraig/laravel-postmark/src/PostmarkTransport.php on line 46

Annoyingly littered over my other beautifully clean logs.