Fix PHP 8.4 implicit nullable deprecation warnings

Problem/Motivation

PHP 8.4 deprecates implicitly nullable parameters. When a parameter has a typed declaration (e.g., array) but has NULL as its default value, PHP 8.4 emits a deprecation warning. In this case:

Deprecated:  Drupal\views_selective_filters\Plugin\views\filter\Selective::init(): Implicitly marking parameter $options as nullable is deprecated, the explicit nullable type must be used instead in /web/modules/contrib/views_selective_filters/src/Plugin/views/filter/Selective.php on line 39

Steps to reproduce

1. Be in a PHP 8.4 environment
2. Create a view
3. Attempt to add a selective filter to the view

Proposed resolution

Add an explicit nullable type declaration to the affected parameter.