Null vendor extensions are excluded from Json/Yaml

[loomisdf]

Description

I haven't been able to create a vendor extension with a null value, the generated yaml/json won't include any. This could be a bug, but maybe there's a workaround I haven't found yet.

Versions

• springboot 2.6.4
• springdocs-openapi 1.6.6
• Java 11

Example Code

@Configuration
public class SwaggerConfig {

    @Bean
    public OpenAPI api() {
        return new OpenAPI()
                .extensions(apiExtensions());
    }

    private Map<String, Object> apiExtensions() {
        var extensions = new HashMap<String, Object>();

        var linkedMap = new LinkedHashMap<String, String>();
        linkedMap.put("property1", "value1");
        linkedMap.put("property2", null);

        extensions.put("x-my-vendor-extensions", linkedMap);

        return extensions;
    }
}

Expected Output

"x-my-vendor-extensions": {
	"property1": "value1",
        "property2": null
}

Actual Output

"x-my-vendor-extensions": {
	"property1": "value1"
}

Null is an acceptable value for vendor extensions according to the openAPI spec: https://swagger.io/docs/specification/openapi-extensions/

The extension value can be a primitive, an array, an object or null

[bnasslahsen]

@loomisdf,

This should completely be fixed at the source in the swagger-core library. Please raise an issue for it.
The OpenApiMixin should include NULL Values: @JsonInclude(value = Include.ALWAYS)

I have only added a workaround:
Use springdoc.writer-with-order-by-keys=true for the next release.