fix : enum has null value, it will be shown

[bejewel-kyoungmin]

Description

openapi: 3.0.0
info:
  title: Sample API
  description: Optional multiline or single-line description in [CommonMark](http://commonmark.org/help/) or HTML.
  version: 0.1.9
servers:
  - url: http://api.example.com/v1
    description: Optional server description, e.g. Main (production) server
  - url: http://staging-api.example.com
    description: Optional server description, e.g. Internal staging server for testing
paths:
  /users:
    get:
      summary: Returns a list of users.
      description: Optional extended description in CommonMark or HTML.
      parameters:
        - in: query
          name: sort
          description: Sort order
          schema:
            type: string
            nullable: true
            enum:
              - asc
              - desc
              - null 
      responses:
        '200':
          description: A JSON array of user names
          content:
            application/json:
              schema: 
                type: string
                nullable: true
                enum:
                  - asc
                  - desc
                  - null 

in this case, in parameters, and response has enum type schema, and enum has null value.
but swagger ui doesn't show null value. it makes reader confused.

Motivation and Context

Fixes #9682

How Has This Been Tested?

I have built swagger ui with fixed version

Screenshots (if appropriate):

before | after

before	after

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[char0n]

@glowcloud please make sure this renders in 2020-12 render as well.

[glowcloud]

Thanks for the changes!

I can confirm that rendering for 2020-12 works fine:

so LGTM!