Implicitly marking a parameter as nullable is deprecated since PHP 8.4.

Problem/Motivation

phpcs detects

FILE: /var/www/html/web/modules/contrib/flexible_permissions/tests/src/Unit/ChainPermissionCalculatorTest.php
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FOUND 0 ERRORS AND 4 WARNINGS AFFECTING 4 LINES
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 291 | 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: $variation_cache.
 292 | 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: $variation_cache_static.
 293 | 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: $cache_static.
 294 | 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_switcher.
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