[BUG][kotlin][client] missing implementation of allowRequiredAsNullable

[4brunu]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

In #4696 was introduced support for OpenApi 3.0 nullable required fields.
There is a flag allowRequiredAsNullable that was introduced to control this feature, but it's not working.

openapi-generator/bin/openapi3/kotlin-client-petstore-nullable-required.sh

Line 29 in 42f685f

ags="generate -i modules/openapi-generator/src/test/resources/3_0/petstore-with-nullable-required.yaml -t modules/openapi-generator/src/main/resources/kotlin-client -g kotlin --artifact-id kotlin-petstore-nullable-required --additional-properties allowRequiredAsNullable=true -o samples/openapi3/client/petstore/kotlin-nullable-required $@"

openapi-generator version

4.2.2

Suggest a fix

Implement the flag to enable or disable this feature.

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[AlexanderEggers]

@4brunu I implemented this feature for the kotlin client. Did I miss something? I am happy to fix any bugs related to this feature.

[4brunu]

@Mordag No It works ok, the problem was that this change changed a lot of things in my project, but after a better look at it, I think the previous generated code was wrong.
Thanks for the contribution by the way 👍