[BUG][Go] Parameter with free-form object type throws NullPointerException

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 version

5.3.1

OpenAPI declaration file content or url

"/settings/integration": {
  "put": {
    "description": "Create or update an instance of an integration",
    "summary": "Create/update an integration instance",
    "operationId": "createUpdateIntegrationInstance",
    "parameters": [
      {
        "name": "CreateIntegrationRequest",
        "in": "body",
        "schema": {
          "type": "object"
        }
      }
    ],
    "responses": {
      "200": {
        "description": "IntegrationWrapper",
        "schema": {
          "$ref": "#/definitions/IntegrationWrapper"
        }
      }
    }
  }
}

Generation Details

Exception in thread "main" java.lang.RuntimeException: Could not generate api file for 'Default'
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:682)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:907)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:441)
	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.GoClientCodegen.constructExampleCode(GoClientCodegen.java:541)
	at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:492)
	at org.openapitools.codegen.languages.GoClientCodegen.postProcessOperationsWithModels(GoClientCodegen.java:460)
	at org.openapitools.codegen.DefaultGenerator.processOperations(DefaultGenerator.java:1224)
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:585)

Start in org.openapitools.codegen.languages.GoClientCodegen.java at constructExampleCode, especially around line 541, following the call from postProcessOperationsWithModels. Reproduce generation with the provided OpenAPI declaration and verify that the Go client is generated without the reported NullPointerException.