Bug: erroneous detection of changes with mediaTypes, failure to generate html/markdown reports

首先，使用提供的 OpenAPI YAML 和命令 java -jar openapi-diff-cli-2.1.2-all.jar anothertest.yaml samefile.yaml --html blah.html --fail-on-incompatible 重現該報告。檢查 mediaTypes 比較以及 HTML/Markdown 報告路徑；當相同的規格不顯示任何變更，且兩種報告格式都能產生而不會出現空指標例外時，即表示完成。

Hello

I've run into an interesting problem with version 2.1.2 of openapi-diff. I've redacted/dumbed down this example spec to be the smallest piece possible.
It would seem that in this case, where the content of then endpoint is uses mediaType's, changes are being detected (though not breaking) but even when the specs are identical.

I boiled the repro to the simplest form, you can take this example, put it in two separate files, and diff them with 2.1.2 (or 2.1.1 - but for instance, 2.1.0.beta-11 does NOT exhibit this)

openapi: '3.0.3'
info:
  license:
    name: ''
  termsOfService: ''
  title: "my test service"
  version: 'Latest'
tags:
  - name: mytestTag
    description: endpoints
paths:
  /v4/mytest/formats/{downloadFormat}/records/{recordId}:
    get:
      tags:
        - mytestTag
      summary: Converts record to provided format
      operationId: getV4DownloadsFormatsRecords
      deprecated: false
      x-ebsco-secured-endpoint:
        requiredScopes: [ ]
      security:
        - Authorization: [ ]
      parameters:
        - name: downloadFormat
          in: path
          description: Export format
          required: true
          schema:
            type: string
          example: 'csv'
        - name: recordId
          in: path
          description: Unique record identifier
          required: true
          example: 'ax74x2wyin'
          schema:
            type: string
      responses:
        '200':
          description: OK
          content:
            text/csv:
              schema:
                type: string
                format: byte
              example: 'blah'
            application/vnd.openxmlformats-officedocument.wordprocessingml.document:
              schema:
                type: string
                format: byte
              example: 'blah'
            application/pdf:
              schema:
                type: string
                format: byte
              example: 'blah'
            application/x-bibtex:
              schema:
                type: string
                format: byte
              example: 'blah'
            application/marc:
              schema:
                type: string
                format: byte
              example: 'blah'
            text/xml:
              schema:
                type: string
                format: byte
              example: 'blah'
        '400':
          $ref: '#/components/responses/BadRequest'
        '401':
          $ref: '#/components/responses/Unauthorized'
        '403':
          $ref: '#/components/responses/Forbidden'
        '404':
          $ref: '#/components/responses/NotFound'
components:
  responses:
    BadRequest:
      description: 'BadRequest'
      content:
        application/json:
          example:
            name: BAD_REQUEST
            details:
              - field: '#/login/username'
                issue: BAD_REQUEST
                location: body
            debugId: '123456789'
            message: Request contains errors
            informationLink: 'http://developer.foo.com/apidoc/blah#BAD_REQUEST'
          schema:
            $ref: '#/components/schemas/ErrorResponse'
    NotFound:
      description: 'NotFound'
      content:
        application/json:
          example:
            name: NOT_FOUND
            details:
              - field: '#/login/username'
                issue: USERNAME_IS_NOT_FOUND
                location: body
            debugId: '123456789'
            message: Item is not found
            informationLink: 'http://developer.foo.com/apidoc/blah#USERNAME_IS_NOT_FOUND'
          schema:
            $ref: '#/components/schemas/ErrorResponse'
    Unauthorized:
      description: 'Unauthorized'
      content:
        application/json:
          example:
            name: UNAUTHORIZED
            details:
              - field: '#/login/username'
                issue: NOT_AUTHORIZED
                location: body
            debugId: '123456789'
            message: User is not authorized
            informationLink: 'http://developer.foo.com/apidoc/blah#NOT_AUTHORIZED'
          schema:
            $ref: '#/components/schemas/ErrorResponse'
    Forbidden:
      description: 'Forbidden'
      content:
        application/json:
          example:
            name: FORBIDDEN
            details:
              - field: '#/login/username'
                issue: FORBIDDEN
                location: body
            debugId: '123456789'
            message: Access denied
            informationLink: 'http://developer.foo.com/apidoc/blah#FORBIDDEN'
          schema:
            $ref: '#/components/schemas/ErrorResponse'
  schemas:
    ErrorResponse:
      type: object
      description: Common Error response
      x-examples:
        example-1:
          name: VALIDATION_ERROR
          details:
            - field: '#/login/username'
              issue: REQUIRED_FIELD_MISSING
              location: body
          debugId: '123456789'
          message: Invalid data provided
          informationLink: 'http://developer.foo.com/apidoc/blah#VALIDATION_ERROR'
  securitySchemes:
    Authorization:
      description: Authorization token
      type: http
      scheme: bearer
      bearerFormat: JWT

This will result in changes being detected, specifically around the mediaTypes

So the detection of changes when the files are identical is curious.. but furthermore, attempting to generate the html or markdown version of the report fails generate with the following exception (for markdown)

Unexpected exception. Reason: Cannot invoke "org.openapitools.openapidiff.core.model.ChangedSchema.isDiscriminatorPropertyChanged()" because "schema" is null

and for html

Unexpected exception. Reason: Cannot invoke "org.openapitools.openapidiff.core.model.ChangedSchema.isIncompatible()" because "schema" is null

When running the command like so:

java -jar openapi-diff-cli-2.1.2-all.jar anothertest.yaml samefile.yaml --html blah.html --fail-on-incompatible