[BUG][Kotlin][Retrofit2] Generator should support optional form parameter

Created by: shanselm-ergon

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

As stated in #8809 (closed) there is a similar problem with the form elements, which can be marked optional, but the code generator does not make them optional.

Experienced:

@Multipart
@POST("do/something")
fun saveWithAttachment(@Part("subject") subject: kotlin.String, @Part file1: MultipartBody.Part): Single<SomethingDTO>

Expected:

@Multipart
@POST("do/something")
fun saveWithAttachment(@Part("subject") subject: kotlin.String? = null, @Part file1: MultipartBody.Part? = null): Single<SomethingDTO>

openapi-generator version

5.1.0

OpenAPI declaration file content or url

"/do/something": {
  "post": {
    "tags": [
      "doSomething"
    ],
    "operationId": "saveWithAttachment",
    "consumes": [
      "multipart/form-data"
    ],
    "produces": [
      "application/json"
    ],
    "parameters": [
      {
        "name": "subject",
        "in": "formData",
        "required": false,
        "type": "string"
      },
      {
        "name": "file1",
        "in": "formData",
        "required": false,
        "type": "file"
      }
    ],
    "responses": {
      "200": {
        "description": "successful operation",
        "schema": {
          "$ref": "#/definitions/SomethingDTO"
        }
      }
    }
  }
}

Related issues/PRs

See #8809 (closed)

Suggest a fix

In formParams.mustache change

{{#isFormParam}}{{^isFile}}{{#isMultipart}}@Part{{/isMultipart}}{{^isMultipart}}@Field{{/isMultipart}}("{{baseName}}") {{{paramName}}}: {{{dataType}}}{{/isFile}}{{#isFile}}{{#isMultipart}}@Part{{/isMultipart}}{{^isMultipart}}@Field("{{baseName}}"){{/isMultipart}} {{{paramName}}}: MultipartBody.Part{{/isFile}}{{/isFormParam}}

to:

{{#isFormParam}}{{^isFile}}{{#isMultipart}}@Part{{/isMultipart}}{{^isMultipart}}@Field{{/isMultipart}}("{{baseName}}") {{{paramName}}}: {{{dataType}}}{{#required}}{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{/required}}{{^required}}?{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{^defaultValue}} = null{{/defaultValue}}{{/required}}{{/isFile}}{{#isFile}}{{#isMultipart}}@Part{{/isMultipart}}{{^isMultipart}}@Field("{{baseName}}"){{/isMultipart}} {{{paramName}}}: MultipartBody.Part{{^required}}? = null{{/required}}{{/isFile}}{{/isFormParam}}