BUG Null-typed extensions elements disappear when creating yaml

When save openapi to yaml ( ObjectMapperFactory.createYaml(true).writeValueAsString(openApi) )

1. some fields (with null type) in extensions disappear
2. in licence section "extensions" word appear, but it shouldn't be there

As far I understand, parsing works correctly with extensions, but creating yaml works with bugs

Full code to reproduce:

import io.swagger.parser.OpenAPIParser;
import io.swagger.v3.core.util.ObjectMapperFactory;

public class Main  {
    public static void main(String[] args) {
        System.out.println("Hello World!");
        OpenAPIParser parser = new OpenAPIParser();
        var result = parser.readContents("""
openapi: 3.1.0
info:
  title: Sample Pet Store App
  contact:
    name: API Support
    url: https://www.example.com/support
    email: support@example.com
    x-myext:
      key1: 1
      key2: null
  license:
    name: asassaas
    x-licence:
      key1: 1
      key2: null
  version: 1.0.1
  summary: A pet store manager.
  x-info:
    key1: 1
    key2: null
    """, null, null);

        var openApi = result.getOpenAPI();

        // now save it as yaml
        var isV31 = true;
        try {
            var yaml = ObjectMapperFactory.createYaml(true).writeValueAsString(openApi);
            System.out.println(yaml);
        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

output is:

openapi: 3.1.0
info:
  title: Sample Pet Store App
  contact:
    name: API Support
    url: https://www.example.com/support
    email: support@example.com
    x-myext:
      key1: 1
  license:
    name: asassaas
    extensions:
      x-licence:
        key1: 1
  version: 1.0.1
  summary: A pet store manager.
  x-info:
    key1: 1
servers:
- url: /

dependencies:

  <dependencies>
    <dependency>
      <groupId>io.swagger.parser.v3</groupId>
      <artifactId>swagger-parser</artifactId>
      <version>2.1.22</version>
    </dependency>

  </dependencies>

Start with the provided reproducer and inspect ObjectMapperFactory.createYaml(true), focusing on serialization of OpenAPI extensions and null-valued entries. Done means the generated YAML preserves extension fields whose values are null and does not emit an unwanted extensions wrapper under license.