mimemailparser: Fix implicitly nullable parameter declarations for PHP 8.4
Summary: Backport upstream's https://github.com/php-mime-mail-parser/php-mime-mail-parser/commit/39633e054b4b3290775c5e9b43d3bb8f47128433 for PHP 8.4 compatibility.
Test Plan: Run static code analysis.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D25840