[BUG] [Kotlin] Kotlin-client with retrofit2 doesn't generate nullable parameters

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

The kotlin-client generator when using Retrofit2 library generates a DefaultApi.kt file that doesn't mark optional parameters as nullable.

openapi-generator version

openapi-generator-cli/4.3.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  version: 0.0.1
  title: Optional Parameter Demo
paths:
  /foo:
    get:
      parameters:
        - in: query
          name: optional
          required: false
          schema:
            type: integer
        - in: query
          name: mandatory
          required: true
          schema:
            type: integer
      responses:
        200:
          description: ''
          content:
            text/plain:
              schema:
                type: integer

Command line used for generation

java -jar .\openapi-generator-cli-4.3.0.jar generate -i ".\foo.yaml" --artifact-id kotlin-client -g kotlin -o generated --library jvm-retrofit2

Steps to reproduce

1. Run the above command on the declaration file provided

Result will be:

package org.openapitools.client.apis

import org.openapitools.client.infrastructure.CollectionFormats.*
import retrofit2.http.*
import retrofit2.Call
import okhttp3.RequestBody
import okhttp3.ResponseBody
import okhttp3.MultipartBody


interface DefaultApi {
    @GET("/foo")
    fun fooGet(@Query("mandatory") mandatory: kotlin.Int, @Query("optional") optional: kotlin.Int): Call<kotlin.Int>

}

Related issues/PRs

#5198
#2012

Suggest a fix

Optional parameter should be @Query("optional") optional: kotlin.Int?

Start with the supplied foo.yaml and run the openapi-generator-cli command using the kotlin generator with the jvm-retrofit2 library. Inspect how DefaultApi.kt is generated for the optional and mandatory query parameters; done means the optional parameter is nullable while the mandatory parameter remains non-nullable.