Failed to parse swagger 2.0 document that contains null value at examples

[filzrev]

Describe the bug

When parsing swagger 2.0 document that contains null value at examples.
It failed to parse document with OpenApiError with message Expected a value and Pointer: null.

It's occurred when using Microsoft.OpenApi.Readers 2.0.0-preview2.
When using 1.6.22, It can successfully parse document without problems.

OpenApi File To Reproduce

{
  "swagger": "2.0",
  "info": {
    "title": "Example",
    "version": "1.0"
  },
  "paths": {
    "/": {
      "get": {
        "responses": {
          "200": {
            "description": "200:OK",
            "examples": {
              "application/json": {
                "dummyValue": null
              }
            }
          }
        }
      }
    }
  },
  "host": "localhost",
  "basePath": "/",
}

Expected behavior
OpenApiDocument.Parse(text).OpenApiDocument returns parsed OpenAPI documents.