PHP 8.4 nullable types must be explicit

Problem/Motivation

There 2 PHP 8.4 deprecated errors in the module.

Steps to reproduce

❯ ~/workspace/php-cs-fixer/vendor/bin/php-cs-fixer check backend/web/modules/contrib/password_policy/ --rules=nullable_type_declaration_for_default_null_value

1) backend/web/modules/contrib/password_policy/password_policy_length/tests/src/Functional/PasswordLengthOperations.php
2) backend/web/modules/contrib/password_policy/src/Plugin/migrate/process/PolicyConfiguration.php

Found 2 of 60 files that can be fixed in 0.203 seconds, 26.50 MB memory used

Proposed resolution

Fix them.

Remaining tasks

Merge request.

User interface changes

none

API changes

none

Data model changes

none