[BUG] Nullref for invalid reference to requestBody on OpenAPI spec

Created by: dmeijboom

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
Have you searched for related issues/PRs?
What's the actual output vs expected output?
[Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

OpenAPI Generator fails with:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.parameters.RequestBody.getContent()" because "requestBody" is null
        at org.openapitools.codegen.InlineModelResolver.flattenRequestBody(InlineModelResolver.java:414)
        at org.openapitools.codegen.InlineModelResolver.flattenPaths(InlineModelResolver.java:143)
        at org.openapitools.codegen.InlineModelResolver.flatten(InlineModelResolver.java:89)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:897)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:465)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

openapi-generator version

6.0.1 (also on master)

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  version: 1.0.0
  title: Example
  license:
    name: MIT
servers:
  - url: http://api.example.xyz/v1
paths:
  /resolve_request_body_invalid_ref:
    post:
      requestBody:
        $ref: '#/components/schemas/Invalid'
      operationId: resolveRequestBodyInvalidRef
      responses:
        '200':
          description: OK
components:
  requestBodies: {}

Generation Details

openapi-generator generate -g rust --skip-validate-spec -i FILE.json -o out

Steps to reproduce

Create an OpenAPI spec
Use an invalid ref in the requestBody

Related issues/PRs

Suggest a fix

PR: #13199