[BUG] [KOTLIN-CLIENT] enums with empty entries to generate invalid kotlin-client code

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

A spec with an empty enum entry no longer generates a valid enum in kotlin code.

openapi-generator version

7.4.0, also 7.5.0

OpenAPI declaration file content or url

openapi: 3.0.2
info:
  title: API
  version: 1.0.0
paths:
  /api/v1/foo/:
    get:
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Foo'
components:
  schemas:
    Foo:
      type: object
      description: foo
      properties:
        field_name:
          nullable: true
          enum:
            - ''
            - stringvalue1
          type: string

Generation Details

will generate this kotlin code:

/**
 *
 * Please note:
 * This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
 * Do not edit this file manually.
 *
 */

@file:Suppress(
    "ArrayInDataClass",
    "EnumEntryName",
    "RemoveRedundantQualifierName",
    "UnusedImport"
)


import com.squareup.moshi.Json
import com.squareup.moshi.JsonClass

/**
 * foo
 *
 * @param fieldName 
 */
@JsonClass(generateAdapter = true)

data class ApiFoo (

    @Json(name = "field_name")
    val fieldName: ApiFoo.FieldName? = null

) {

    /**
     * 
     *
     * Values: ,stringvalue1
     */
    @JsonClass(generateAdapter = false)
    enum class FieldName(val value: kotlin.String) {
        @Json(name = "") (""),
        @Json(name = "stringvalue1") stringvalue1("stringvalue1");
    }
}

which has the invalid first entry. Previously this (in 7.3.0) this would generate the following:

/**
 *
 * Please note:
 * This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
 * Do not edit this file manually.
 *
 */

@file:Suppress(
    "ArrayInDataClass",
    "EnumEntryName",
    "RemoveRedundantQualifierName",
    "UnusedImport"
)


import com.squareup.moshi.Json
import com.squareup.moshi.JsonClass

/**
 * foo
 *
 * @param fieldName 
 */
@JsonClass(generateAdapter = true)

data class ApiFoo (

    @Json(name = "field_name")
    val fieldName: ApiFoo.FieldName? = null

) {

    /**
     * 
     *
     * Values: eMPTY,stringvalue1
     */
    @JsonClass(generateAdapter = false)
    enum class FieldName(val value: kotlin.String) {
        @Json(name = "") eMPTY(""),
        @Json(name = "stringvalue1") stringvalue1("stringvalue1");
    }
}

with the eMPTY entry.

here is my openApiGenerate task

openApiGenerate {
  generatorName = "kotlin"
  inputSpec = specFile
  outputDir = generationDir
  templateDir = "$templatesDir"
  apiPackage = "com.giftster.network.api"
  packageName = "com.giftster.network"
  cleanupOutput = true
  modelNamePrefix = "Api"
  library = "jvm-retrofit2"
  generateApiTests = false
  generateModelTests = false
  configOptions.set([
    artifactId              : "client",
    dateLibrary             : "java8",
    groupId                 : "com.foo,
    moshiCodeGen            : "true",
    omitGradlePluginVersions: "true",
    omitGradleWrapper       : "true",
    useCoroutines           : "true"
  ])
}

Steps to reproduce

Run the generation on the yaml.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/18062

Suggest a fix

Should revert the enum generation changes or provide guidance on fixing the breaking change.

Start with the supplied OpenAPI YAML and reproduce the output using the Kotlin generator at version 7.4.0 or 7.5.0. Review the enum-generation changes in related PR #18062 and compare them with the 7.3.0 output. Done means an empty enum value produces valid Kotlin code while preserving its empty serialized value.