PHP 8.4 implicit nullable deprecations

Problem/Motivation

Errors upon update to PHP 8.4:

Deprecated function: Drupal\user_mailman_register\Plugin\views\filter\ListDescriptions::init(): Implicitly marking parameter $options as nullable is deprecated, the explicit nullable type must be used instead in include() (line 576 of /path/to/vendor/composer/ClassLoader.php) 

Deprecated function: Drupal\user_mailman_register\Form\AdminDeleteForm::buildForm(): Implicitly marking parameter $list as nullable is deprecated, the explicit nullable type must be used instead in include() (line 576 of /path/to/vendor/composer/ClassLoader.php).

Proposed resolution

Fix the type hints in those two files.

User interface changes

None

API changes

None

Data model changes

None