required and nullable repro

openapi.yaml

openapi: 3.0.1
info:
  title: test API
  version: 2.0.0
  license:
    name: Apache 2.0
    url: 'http://www.apache.org/licenses/LICENSE-2.0.html'
servers:
  - url: '/api/v1/'
paths:
  '/cat':
    post:
      operationId: createCat
      requestBody:
        content:
          application/json:
            schema:
              "$ref": "#/components/schemas/Cat"
      responses:
        "201":
          description: Created
          content:
            application/json:
              schema:
                "$ref": "#/components/schemas/Cat"

components:
  schemas:
    Cat:
      type: object
      properties:
        id:
          type: string
          nullable: true
      required:
      - id