[BUG] Parsing Remote Reference (Filesystem) openapi 3.0 yaml File produces NPE

Description

I am getting JsonParseException while parsing Remote Reference (Filesystem) yaml File ($ref: './model.yaml#/components/schemas/Model')

openapi-generator version

4.3.0

openapi.yaml

openapi: "3.0.0"
info:
  version: 1.0.0
  title: TODO
servers:
  - url: TODO
paths:
  /konten/method3:
    post:
      operationId: javaMethodName
      responses:
        '200':
          content:
            application/json:
              schema:
                $ref: './model.yaml#/components/schemas/Model'

model.yaml (no openapi: 3.0.0 on 1st line)

components:
  schemas:
    Model:
      type: object
      properties:
        integerMember:
          type: integer
          format: int32

Both files are located in the same directory

It seems that the Message "attribute openapi is missing" in OpenAPIV3Parser causes the problem!

Start with OpenAPIV3Parser and reproduce the issue using the provided openapi.yaml and model.yaml files, with the filesystem $ref and no openapi field in model.yaml. Trace how the missing openapi attribute is handled during remote-reference parsing. Done means the referenced schema parses without a JsonParseException or NullPointerException.