PHP 8.4.x deprecation warnings

检查 SpecBaseObject.php、SpecObjectInterface.php、spec/Paths.php、spec/PathItem.php 和 spec/Responses.php 中的 resolveReferences 签名，以及 spec/Reference.php 中的构造函数和 resolve 方法。更新警告中识别出的隐式 nullable 参数，并在 PHP 8.4 下验证这些弃用警告不再出现。

With the release of PHP 8.4, implicitly marking parameters as nullable is now deprecated.

The dev-master version of this package contains a few warnings because of this:

<warning> DEPRECATED </warning> cebe\openapi\SpecBaseObject::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/SpecBaseObject.php on line 396.
<warning> DEPRECATED </warning> cebe\openapi\SpecObjectInterface::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/SpecObjectInterface.php on line 43.
<warning> DEPRECATED </warning> cebe\openapi\spec\Paths::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Paths.php on line 249.
<warning> DEPRECATED </warning> cebe\openapi\spec\PathItem::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/PathItem.php on line 153.
<warning> DEPRECATED </warning> cebe\openapi\spec\Responses::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Responses.php on line 239.
<warning> DEPRECATED </warning> cebe\openapi\spec\Reference::__construct(): Implicitly marking parameter $to as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Reference.php on line 72.
<warning> DEPRECATED </warning> cebe\openapi\spec\Reference::resolve(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Reference.php on line 179.
<warning> DEPRECATED </warning> cebe\openapi\spec\Reference::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Reference.php on line 366.

Example:

public function resolveReferences(ReferenceContext $context = null); // SpecObjectInterface.php #43