Description in Response cannot be null

[StefH]

Describe the bug
According to the spec, the description cannot be null.
https://spec.openapis.org/oas/v3.0.4.html

OpenApi File To Reproduce

var doc = new OpenApiDocument
{
    Paths = new OpenApiPaths
    {
        ["/example"] = new OpenApiPathItem
        {
            Operations = new Dictionary<HttpMethod, OpenApiOperation>
            {
                [HttpMethod.Get] = new OpenApiOperation
                {
                    Responses = new OpenApiResponses
                    {
                        ["200"] = new OpenApiResponse
                        {
                            // No description
                        }
                    }
                }
            }
        }
    }
};

using var stringWriter = new StringWriter();
doc.SerializeAs(OpenApiSpecVersion.OpenApi3_0, new OpenApiJsonWriter(stringWriter));
var result = stringWriter.ToString();

Expected behavior

{
  "openapi": "3.0.4",
  "info": { },
  "paths": {
    "/example": {
      "get": {
        "responses": {
          "200": {
          }
        }
      }
    }
  }
}

Actual behavior

{
  "openapi": "3.0.4",
  "info": { },
  "paths": {
    "/example": {
      "get": {
        "responses": {
          "200": {
            "description": null
          }
        }
      }
    }
  }
}

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I used latest NuGet.

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Swagger: