PHP 8.4 implicit nullable deprecation

Problem/Motivation

PHP 8.4 has been released, and this module is triggering a "use explicit nullable type" deprecation.

This is part of Drupal's coding standards now, but since the issues to fix phpcs in this module have stalled, I'm opening up this issue to fix this single deprecation to avoid the warnings on PHP 8.4.

After updating to PHP 8.4:

FILE: /src/Plugin/Action/UnpublishByKeywordComment.php
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FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
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 123 | WARNING | Implicitly marking a parameter as nullable is deprecated since PHP 8.4. Update the type to be explicitly nullable instead. Found implicitly nullable parameter: $account.
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FILE: /src/Plugin/Action/AssignOwnerNode.php
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FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
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 139 | WARNING | Implicitly marking a parameter as nullable is deprecated since PHP 8.4. Update the type to be explicitly nullable instead. Found implicitly nullable parameter: $account.
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FILE: /src/Plugin/Action/UnpublishByKeywordNode.php
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FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
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 73 | WARNING | Implicitly marking a parameter as nullable is deprecated since PHP 8.4. Update the type to be explicitly nullable instead. Found implicitly nullable parameter: $account.
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FILE: /tests/action_form_ajax_test/src/Plugin/Action/ActionAjaxTest.php
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FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
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 34 | WARNING | Implicitly marking a parameter as nullable is deprecated since PHP 8.4. Update the type to be explicitly nullable instead. Found implicitly nullable parameter: $account.
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