[BUG] [Golang] JSON unmarshal fails on additional properties when additionalProperties is unset

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

When a model lists required fields, but additionalProperties is not set, it should treat it as being set to true, but instead the JSON parsing fails with an error like.

json: unknown field "moreStuff"

openapi-generator version

7.12.0

OpenAPI declaration file content or url

Minimal OpenAPI spec to reproduce (spec.yaml):

openapi: 3.0.0
info:
  title: Sample API
  description: Minimal spec
  version: 0.0.1

servers:
  - url: http://localhost:8080

paths:
  /something:
    get:
      summary: Returns something
      responses:
        "200":
          description: A JSON array of somethings
          content:
            application/json:
              schema:
                type: object
                properties:
                  stuff:
                    type: string
                required:
                - stuff

Generation Details

openapi-generator-cli generate -g go -i spec.yaml

Steps to reproduce

Run web server serving problematic (but not incompatible with the spec!) JSON, like the following (server.go):

package main

import (
	"encoding/json"
	"net/http"
)

type Response struct {
	Stuff     string `json:"stuff"`
	MoreStuff string `json:"moreStuff"`
}

func handler(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "application/json")
	response := Response{Stuff: "something", MoreStuff: "something else"}
	json.NewEncoder(w).Encode(response)
}

func main() {
	http.HandleFunc("/something", handler)
	http.ListenAndServe(":8080", nil)
}

using a command like

go run server.go

After generating the client, remove the skip in line ~27 in the test file test/api_default_test.go,
onstall deps and run the test

go mod tidy
go test ./...

Error:      	Expected nil, but got: &openapi.GenericOpenAPIError{[...], error:"json: unknown field \"moreStuff\"", model:interface {}(nil)}

Adding additionalProperties: true to the spec like following makes the test pass:

openapi: 3.0.0
info:
  title: Sample API
  description: Minimal spec
  version: 0.0.1

servers:
  - url: http://localhost:8080

paths:
  /something:
    get:
      summary: Returns something
      responses:
        "200":
          description: A JSON array of somethings
          content:
            application/json:
              schema:
                type: object
                additionalProperties: true
                properties:
                  stuff:
                    type: string
                required:
                - stuff

Related issues/PRs

#17267 added the validation of unknown properties when required fields are set but additionalProperties is not.

Suggest a fix

I think the problem is that an empty additionalProperties field is not being treated equal to being set to true. As far as I understand that's how it should be handled, though.

Reproduce the issue using spec.yaml, the generated Go client, and test/api_default_test.go after removing the skip; start by reviewing the unknown-property validation introduced by related issue #17267. Done means the generated client accepts extra JSON properties when additionalProperties is omitted, while preserving validation for required fields.