Issue #3591979: Add explicit nullable Webform submission types

Closes #3591979

This updates the optional WebformSubmissionInterface parameters in WebformRemoteSelectElement to use explicit nullable types for PHP 8.4 compatibility.

This follows the patch attached by luigisa and keeps the behavior unchanged.

Checked locally: php -l on all PHP files git diff --check

There is no composer.json in this module checkout, so I could not run composer-based checks locally.