[Java] useNullForUnknownEnumValue

Created by: nvierge

Description

The option useNullForUnknownEnumValue exists but does not appear when using config-help to list config options. I find an example that use this option and the syntax is not standard: --additional-properties useNullForUnknownEnumValue=true

Is it a voluntary choice ?

openapi-generator version

3.3.1

Related issues/PRs

PR #633

Suggest a fix/enhancement

Add this option in config options.