[BUG] [Kotlin] Enum with 'null' case no longer generate valid code

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

Starting from OpenAPI Generator release 7.9.0, a schema with a single enum case named "null" no longer generate compilable code.

The enum class generated do not include the case named:

@JsonClass(generateAdapter = false)
enum class NullValueKO(val value: kotlin.String) {

    /**
     * Override [toString()] to avoid using the enum variable name as the value, and instead use
     * the actual value defined in the API spec file.
     *
     * This solves a problem when the variable name and its value are different, and ensures that
     * the client sends the correct enum values to the server always.
     */
    override fun toString(): kotlin.String = value

    companion object {
[...]

In 7.8.0 the enum class generated was:

enum class NullValue(val value: kotlin.String) {

    @JsonProperty(value = "null")
    `null`("null");

    /**
     * Override [toString()] to avoid using the enum variable name as the value, and instead use
     * the actual value defined in the API spec file.
     *
     * This solves a problem when the variable name and its value are different, and ensures that
     * the client sends the correct enum values to the server always.
     */
    override fun toString(): kotlin.String = value
[...]

openapi-generator version

This a regression starting from 7.9.0 up to 7.13.0

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Test NullValue enum case generation
  version: 1.0.0
paths:
components:
  schemas:
    NullValueKO:
      type: string
      enum:
        - Null
      nullable: true
    NullValueOK:
      type: string
      enum:
        - MyNull
      nullable: true

Generation Details

This is the command line used to generate the code:

openapi-generator generate -i input/nullvalue.yaml -g kotlin -a library=retrofit2  -o output/kotlin --skip-validate-spec

Steps to reproduce

Use the provide command line given in "Generation Details" and the yaml from "OpenAPI declaration file content or url"

Related issues/PRs

Suggest a fix

Start with input/nullvalue.yaml and run the documented openapi-generator command using the kotlin generator and retrofit2 library. Compare the generated NullValueKO enum with the 7.8.0 example; done means the enum includes the null case and produces compilable Kotlin code.