PHP Deprecation warning on PHP 8.4

Problem/Motivation

Using the module with PHP8.4 generates a deprecation notice:

Deprecated: Drupal\advban\Form\AdvbanEdit::buildForm(): Implicitly marking parameter $ban_id as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/advban/src/Form/AdvbanEdit.php on line 72

Proposed resolution

Update the code to explicitly mark nullable parameters.