[BUG][kotlin-server] required nullable properties generated incorrectly

[dennisameling]

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

When having a required and nullable property in my OpenAPI spec, it becomes non-nullable in the Kotlin code.

Taking the YAML example later down in this bug report, it results in this Kotlin code with the kotlin-server generator:

package com.example.models

/**
 * 
 * @param notNullableRequired 
 * @param nullableRequired 
 * @param nullableNotRequired 
 * @param notNullableNotRequired 
 */
data class Pet(
    val notNullableRequired: kotlin.String,
    val nullableRequired: kotlin.String,
    val nullableNotRequired: kotlin.String? = null,
    val notNullableNotRequired: kotlin.String? = null
) 

The nullableRequired example is incorrect, because it's supposed to be nullable. It should be kotlin.String? instead.

openapi-generator version

7.8.0

OpenAPI declaration file content or url

# OpenAPI schemas support 4 possible combinations for properties:
# 1. nullable + required
# 2. nullable + not required
# 3. not nullable + required
# 4. not nullable + not required
# This sample contains all of them to demonstrate how they are rendered in the generated code.
openapi: 3.0.3
info:
  title: Nullable Required
  version: 1.0.0
servers:
  - url: /api/v3
paths:
  /pet:
    post:
      operationId: addPet
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Pet'
      responses:
        "200":
          description: Successful operation
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Pet'
components:
  schemas:
    Pet:
      required:
        - notNullable_required
        - nullable_required
      type: object
      properties:
        nullable_notRequired:
          type: string
          example: doggie
          nullable: true
        notNullable_notRequired:
          type: string
          example: doggie
        notNullable_required:
          type: string
          example: doggie
        nullable_required:
          type: string
          example: doggie
          nullable: true

Generation Details

Using the openapi-generator Gradle plugin:

openApiGenerate {
    group = "build"
    generatorName = "kotlin-server"
    inputSpec = "src/main/resources/openapi/example.yaml"
    outputDir = "${layout.buildDirectory.get()}/generated/example"
    packageName = "com.example"
    library = "javalin6"
}

Steps to reproduce

• Take the YAML above and run the OpenAPI Generator (Gradle or CLI) for the kotlin-server generator

Related issues/PRs

Probably related issue: #14765

Suggest a fix

I'll provide a PR right after submitting this with a fix. It's a bug in the Mustache template.