[PHP 8.4] Fix implicitly nullable type declarations

Problem/Motivation

In PHP 8.4, declaring functions/methods with parameters containing null as a default value, but without null as one of the types (either as a nullable syntax or as a Union type with null) is deprecated.

https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullabl...

Steps to reproduce

phpcs --standard=SlevomatCodingStandard --sniffs=SlevomatCodingStandard.TypeHints.NullableTypeForNullDefaultValue

FILE: /app/docroot/modules/contrib/allowed_languages/src/AllowedLanguagesManagerInterface.php
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FOUND 3 ERRORS AFFECTING 3 LINES
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 25 | ERROR | [x] Parameter $account has null default value, but is not marked as nullable.
 36 | ERROR | [x] Parameter $account has null default value, but is not marked as nullable.
 49 | ERROR | [x] Parameter $account has null default value, but is not marked as nullable.
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PHPCBF CAN FIX THE 3 MARKED SNIFF VIOLATIONS AUTOMATICALLY
---------------------------------------------------------------------------------------------


FILE: /app/docroot/modules/contrib/allowed_languages/src/AllowedLanguagesManager.php
--------------------------------------------------------------------------------------------
FOUND 3 ERRORS AFFECTING 3 LINES
--------------------------------------------------------------------------------------------
 49 | ERROR | [x] Parameter $account has null default value, but is not marked as nullable.
 66 | ERROR | [x] Parameter $account has null default value, but is not marked as nullable.
 87 | ERROR | [x] Parameter $account has null default value, but is not marked as nullable.
--------------------------------------------------------------------------------------------
PHPCBF CAN FIX THE 3 MARKED SNIFF VIOLATIONS AUTOMATICALLY
--------------------------------------------------------------------------------------------