[BUG] [Go] Optional array properties are rendered as values instead of pointers

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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 creating a schema that contains an optional field of type array, the resulting code will always generate a slice type in the resulting Go code.

This makes it impossible to differentiate between input that:

• contains the array property, with an empty list
• does not contain the array property

Adding nullable: true has no influence on the result. Adding an indirect schema that wraps the array type has no effect either.

openapi-generator version

7.16.0

OpenAPI declaration file content or url

---
openapi: 3.0.0
info:
  title: unset
  version: "1"
paths: {}
components:
  schemas:
    Bar:
      type: object
      properties:
        X:
          type: integer
        Y:
          type: string
        Z:
          # has no influence on the result:
          # nullable: true
          type: array
          items:
            $ref: "#/components/schemas/Baz"
      required:
        - X
    Baz:
      type: string

Generation Details

No special options are used.

The CLI was called as follows:

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

Steps to reproduce

Run the CLI as described above on the provided template.

This produces a model like this:

// Bar struct for Bar
type Bar struct {
        X int32 `json:"X"`
        Y *string `json:"Y,omitempty"`
        Z []string `json:"Z,omitempty"`
}

My expectation would be that Z has the type *[]string instead.

Related issues/PRs

There were several discussions about how to handle optional and nullable properties in the past (most prominently: https://github.com/OpenAPITools/openapi-generator/issues/522), but it looks like handling of array types was not implemented in the same way as for other types.

I tested a few older versions of openapi-generator, and found that 4.3.1 does indeed produce a pointer type (*[]string). So this seems like a regression to me, and that it was simply forgotten when handling of optional properties was overhauled in the Go generator. Ironically, 4.3.1 doesn't produce a pointer type on property Y.

Suggest a fix

Start by running the stated openapi-generator-cli command with the provided openapi.yaml and the Go generator, then inspect the generated Bar model. Trace how the optional array property Z is represented and compare it with the expected *([]string) output. Done means optional arrays preserve the distinction between an absent property and an empty list.