Fix PHP 8.4 deprecation warnings

-Deprecated: Drupal\change_pwd_page\Form\ChangePasswordForm::buildForm(): Implicitly marking parameter $user as nullable is deprecated, the explicit nullable type must be used instead in /app/web/modules/contrib/change_pwd_page/src/Form/ChangePasswordForm.php on line 68

Resolved the above by explicitly marking $user as nullable with a "?"