Fix PHP 8.4 deprecation: use explicit nullable types instead of implicit.

Johnrequested to merge
issue/background_image-3573932:3573932-php-8.4-compatibility into 2.0.x

PHP 8.4 deprecates implicitly marking parameters as nullable via a NULL default value. Add the explicit ? nullable type prefix to all 9 affected parameter declarations across the module.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

Closes #3573932

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