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?

Description

It looks like #8040 broke the ruby client generator when required properties point to a oneOf schema.

openapi-generator version

master @ acedd1c

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: oneOf repro
  version: v1
paths:
  '/hello':
    post:
      operationId: hello.post
      requestBody:
        content:
          application/json:
            schema:
              type: object
              required: [greeting]
              properties:
                greeting:
                  $ref: '#/components/schemas/Greeting'
      responses:
        '204':
          description: returns a greeting

components:
  schemas:
    Greeting:
      oneOf:
        - type: string
        - type: object

Generation Details

openapi-generator generate -i oneof_broken.yaml -g ruby --library faraday -o out/ruby_oneof

Steps to reproduce

1. run the command above by passing the openapi file yet above
2. generation fails with

   Exception in thread "main" java.lang.RuntimeException: Could not generate api file for 'Default'
   	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:675)
   	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:878)
   	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:440)
   	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
   	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
   Caused by: java.lang.NullPointerException
   	at org.openapitools.codegen.languages.RubyClientCodegen.constructExampleCode(RubyClientCodegen.java:736)
   	at org.openapitools.codegen.languages.RubyClientCodegen.constructExampleCode(RubyClientCodegen.java:751)
   	at org.openapitools.codegen.languages.RubyClientCodegen.constructExampleCode(RubyClientCodegen.java:726)
   	at org.openapitools.codegen.languages.RubyClientCodegen.constructExampleCode(RubyClientCodegen.java:759)
   	at org.openapitools.codegen.languages.RubyClientCodegen.constructExampleCode(RubyClientCodegen.java:664)
   	at org.openapitools.codegen.languages.RubyClientCodegen.postProcessOperationsWithModels(RubyClientCodegen.java:588)
   	at org.openapitools.codegen.DefaultGenerator.processOperations(DefaultGenerator.java:1194)
   	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:580)
   

3. comment out the required: [greeting] line
4. generation works

Related issues/PRs

#8040

Suggest a fix

unsure, sorry.

@zippolyte @jirikuncar @wing328