Add support for empty Security Requirement Object (`{}`) in security requirement

From https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.4.md#security-requirement-object

An empty Security Requirement Object (`{}`) indicates anonymous access is supported.

This is used in https://raw.githubusercontent.com/cloudflare/api-schemas/refs/heads/main/openapi.yaml L164315


With current lib and above Cloudflare spec we get error:

```
There was 1 error:

1) IssueTest::test224FailsOnLargeDefinitions
Undefined offset: 0

/app/src/spec/SecurityRequirements.php:29
/app/src/SpecBaseObject.php:168
/app/src/SpecBaseObject.php:140
/app/src/SpecBaseObject.php:168
/app/src/SpecBaseObject.php:140
/app/src/spec/PathItem.php:81
/app/src/spec/Paths.php:62
/app/src/SpecBaseObject.php:168
/app/src/SpecBaseObject.php:140
/app/src/Reader.php:57
/app/src/Reader.php:138
/app/tests/IssueTest.php:17
phpvfscomposer:///app/vendor/phpunit/phpunit/phpunit:106

```

Observed this issue while fixing https://github.com/cebe/php-openapi/issues/224

So adding implementation for empty Security Requirement Object (`{}`) in security requirement is required.

More info about security schemes and security requirements can be obtained at https://swagger.io/docs/specification/v3_0/authentication/

Bắt đầu trong spec/SecurityRequirements.php, nơi xảy ra Undefined offset được báo cáo, và xem xét tests/IssueTest.php, bao gồm test224FailsOnLargeDefinitions. Tái hiện trường hợp Cloudflare OpenAPI và xác nhận rằng một Security Requirement Object rỗng được chấp nhận mà không có lỗi đã báo cáo.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.