[REQ] Enable to Set null to enum value

Is your feature request related to a problem? Please describe.

Hello. We're using openapi-generator for defining our interface.
We want to allow null to enum values.

animal:
    type: string
    nullable: true
    enum: 
        - "1"
        - "2"
        - null
     x-enum-varnames:
        - "CAT"
        - "DOG"
        - "HORSE"

generated code

enum class Animal(val value: kotlin.String) {
     @JsonProperty("1") CAT("1"),
     @JsonProperty("2") DOG("2"),
     @JsonProperty("null") HORSE("null"),
}

Describe the solution you'd like

we want something like

enum class Animal(val value: kotlin.String) {
     @JsonProperty("1") CAT("1"),
     @JsonProperty("2") DOG("2"),
     @JsonProperty(null) HORSE(null),
}

Describe alternatives you've considered

replace "null" to null by hand...

Additional context

If this feature can be supported, I am eager to make pull request
(however, I'm not sure where to fix by now...)

Thanks.

Start by tracing how the OpenAPI schema's nullable enum is represented during Kotlin enum generation. Compare the requested generated Kotlin output with the current handling of the null enum value, then add coverage for the provided Animal schema and verify that the generated annotation and enum value preserve null rather than the string "null".