[BUG][Go] Remove "null" body value when body is empty

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

If you want to close the response with no body, the generated code sends null as body.
For example, you need to create a new user with name and firstname, if everything is ok, the user is created and the api wants to return 201 - Created with no body.

• The current output is:
  

• The expected output would be:
  

openapi-generator version

Tool: @openapitools/openapi-generator-cli@2.5.2

openapi-generator-cli 6.2.1
  commit : b0ce532 
  built  : 2022-11-01
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  description: Users API
  version: 1.0.0
  title: Basic API
  contact:
    email: rcebrian@github.com
  license:
    name: Apache 2.0
    url: 'https://www.apache.org/licenses/LICENSE-2.0.html'
servers:
  - url: 'http://localhost:8080'
    description: localhost
tags:
  - name: users
    description: Users use cases
paths:
  /users:
    post:
      summary: Create user
      operationId: createUser
      responses:
        '201':
          description: Created
        '400':
          description: Bad Request
          content:
            application/json:
              schema:
                type: object
                properties:
                  errors:
                    type: string
        '500':
          description: Internal Server Error
      description: Save user into data storage
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/User'
      tags:
        - users
components:
  schemas:
    User:
      title: User
      type: object
      description: ''
      properties:
        id:
          type: string
        name:
          type: string
        firstname:
          type: string
      required:
        - name
        - firstname
  responses: {}

Generation Details

• Configuration file (config.yaml):

outputAsLibrary: true
addResponseHeaders: true
sourceFolder: openapi
packageName: server
serverPort: 8080

SPECS_FILE="api/openapi-spec/openapi.yaml"
CONFIG_FILE="api/openapi-spec/config.yaml"
OUTPUT_DIR="internal/platform/server"


export GO_POST_PROCESS_FILE="goimports -w"
openapi-generator-cli generate --generator-name go-server \
  --input-spec $SPECS_FILE \
  --config $CONFIG_FILE \
  --global-property apiDocs=true \
  --global-property verbose=false \
  --model-name-suffix dto \
  --enable-post-process-file \
  -o $OUTPUT_DIR

Steps to reproduce

• Start the server on port 8080
• MAke request
  • Method: POST
  • localhost:8080/users
  • Body: { "name": "John", "firstname": "Doe" }

Related issues/PRs

.

Suggest a fix

Edit routers.go template, on method EncodeJSONResponse encode body if not null, nothing otherwise

Start with the routers.mustache template, specifically the EncodeJSONResponse method named in the report. Generate the go-server output using the supplied OpenAPI specification and config.yaml, then reproduce the POST /users response. Done means an empty 201 response no longer sends a null body.