[BUG] [GOLANG] Nil property fields

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

I noticed a change while using docker image openapitools/openapi-generator-cli:v5.2.1 => openapitools/openapi-generator-cli:v5.3.0.

What is expected behaviour? In docker image version 5.3.0 I want to generate following code State *ExampleState `json:"state,omitempty"

openapi-generator version

openapitools/openapi-generator-cli:v5.2.1
vs
openapitools/openapi-generator-cli:v5.3.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Example
  version: '0.1'
  description: |-
servers:
  - url: 'https://example.com'
    description: prod
paths:
  /example:
    get:
      operationId: example
      summary: example
      description: GO *pointers behaves differently, models is different between versions
      responses:
        '200':
          $ref: '#/components/responses/ExampleResponse'
components:
  schemas:
    Example:
      title: Example
      type: object
      description: ''
      properties:
        id:
          type: string
          format: uuid
          readOnly: true
          description: ''
        state:
          readOnly: true
          allOf:
            - $ref: '#/components/schemas/ExampleState'
      required:
        - id
    ExampleState:
      title: ExampleState
      type: string
      description: ''
      enum:
        - A
        - B
        - C
  responses:
    ExampleResponse:
      description: A list of examples
      content:
        application/json:
          schema:
            title: Examples
            type: object
            description: Response structure of the list examples request
            properties:
              examples:
                type: array
                items:
                  $ref: '#/components/schemas/Example'
    Example:
      description: Example response
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/Example'

Generation Details

Steps to reproduce

docker run --rm
--volume "/Users/xx/openapi-generator-cli:/local"
--user "1001:121"
openapitools/openapi-generator-cli:v5.2.1
generate
--input-spec /local/openapi.yaml
--global-property models,modelDocs=false
--generator-name go
--additional-properties packageName=models
--output /local/models

v5.2.1 Generate:
// Example struct for Example
type Example struct {
Id string json:"id"
State *ExampleState json:"state,omitempty"
}

docker run --rm
--volume "/Users/xx/openapi-generator-cli:/local"
--user "1001:121"
openapitools/openapi-generator-cli:v5.3.0
generate
--input-spec /local/openapi.yaml
--global-property models,modelDocs=false
--generator-name go
--additional-properties packageName=models
--output /local/models

v5.3.0 Generate:
// Example struct for Example
type Example struct {
Id string json:"id"
State ExampleState json:"state,omitempty"
}

Suggest a fix

Start with the supplied OpenAPI declaration and the docker generate commands for the Go generator, comparing the generated Example model between v5.2.1 and v5.3.0. Done means the state field is generated with the expected pointer type while preserving the reproduction's other output.