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/

報告された Undefined offset が発生する spec/SecurityRequirements.php から始め、test224FailsOnLargeDefinitions を含む tests/IssueTest.php を確認してください。Cloudflare OpenAPI のケースを再現し、空の Security Requirement Object が報告されたエラーなしで受け入れられることを確認してください。

索引モデルが issue の本文から書いたものです。