PHP 8.4: fix implicit nullable deprecations [#3543564]

Problem/Motivation

PHP 8.4 introduces a deprecation for implicitly nullable parameters: https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullabl...

Steps to reproduce

Install module on Drupal 10.5 with PHP 8.4. Log shows deprecation notices such as:

PHP Deprecated: Drupal\flag_search_api\Plugin\search_api\processor\FlagIndexer::getPropertyDefinitions(): Implicitly marking parameter $datasource as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/flag_search_api/src/Plugin/search_api/processor/FlagIndexer.php on line 120

Proposed resolution

Adjust the nullable parameters to declare them as nullable.

Issue fork flag_search_api-3543564

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