minor #54906 PHP 8.4 nullability deprecation (sam-bee)

This PR was merged into the 7.1 branch.

Discussion
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PHP 8.4 nullability deprecation

### PHP 8.4 deprecation of implicit nullability.

| Q             | A
| ------------- | ---
| Branch?       | 7.1
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        |
| License       | MIT

Because PHP 8.4 is adding deprecation warnings for non-nullable parameters with null default, change typehints.

This relates to the [implicit nullability RFC](https://wiki.php.net/rfc/deprecate-implicitly-nullable-types) in PHP 8.4.

Commits
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8c3511b Because PHP 8.4 is adding deprecation warnings for non-nullable parameters with null default, change typehints

src/Symfony/Component/Validator/Validation.php

@@ -40,7 +40,7 @@ public static function createCallable(Constraint|ValidatorInterface|null $constr
     /**
      * Creates a callable that returns true/false instead of throwing validation exceptions.
      *
-     * @return callable(mixed $value, ConstraintViolationListInterface &$violations = null): bool
+     * @return callable(mixed $value, ?ConstraintViolationListInterface &$violations = null): bool
      */
     public static function createIsValidCallable(Constraint|ValidatorInterface|null $constraintOrValidator = null, Constraint ...$constraints): callable
     {

src/Symfony/Contracts/HttpClient/Test/TestHttpServer.php

@@ -21,7 +21,7 @@ class TestHttpServer
     /**
      * @param string|null $workingDirectory
      */
-    public static function start(int $port = 8057/* , string $workingDirectory = null */): Process
+    public static function start(int $port = 8057/* , ?string $workingDirectory = null */): Process
     {
         $workingDirectory = \func_get_args()[1] ?? __DIR__.'/Fixtures/web';