Fix PHP 8.4 deprecations #109
Description
Running this module on PHP 8.4 will generate the following deprecation notices.
Exercise\HTMLPurifierBundle\DependencyInjection\ExerciseHTMLPurifierExtension::getResolvedConfig(): Implicitly marking parameter $definition as nullable is deprecated, the explicit nullable type must be used instead
Exercise\HTMLPurifierBundle\CacheWarmer\SerializerCacheWarmer::warmUp(): Implicitly marking parameter $buildDir as nullable is deprecated, the explicit nullable type must be used instead
The fix is to explictly mark them as nullable (so string = null becomes ?string = null). This syntax was introduced in PHP 7.1 so there are no compatibility concerns as the bundle already requires PHP 8.1 or above.