[DefaultGenerator] NullPointerException when no components defined

Created by: jmini

Description

When there isn't any component defined in the specification, there is a NullPointerException in org.openapitools.codegen.utils.ModelUtils.getUnusedSchemas(OpenAPI)

Exception in thread "main" java.lang.NullPointerException
	at org.openapitools.codegen.utils.ModelUtils.getUnusedSchemas(ModelUtils.java:96)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:781)
	at fr.jmini.openapi.openapitools.generator.TestOpenapitoolsCodegenMain.convert(TestOpenapitoolsCodegenMain.java:79)

openapi-generator version

3.0.0-SNAPSHOT

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: ping test
  version: '1.0'
servers:
  - url: 'http://localhost:8000/'
paths:
  /ping:
    get:
      operationId: pingGet
      responses:
        '201':
          description: OK