[BUG] Kotlin generated code makes non-nullable shema fields nullable

Bug Report Checklist

• [x ] Have you provided a full/minimal spec to reproduce the issue?
• [ x] Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• [x ] Have you searched for related issues/PRs?
• [ x] What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When I use the open-api-generator all of the model fields are ? nullable, all classes look like this:

data class PagedResultDtoOfStoredDeviceFilterWithoutTermDto (

    @Json(name = "currentPage")
    val currentPage: kotlin.Int? = null,

    @Json(name = "totalPages")
    val totalPages: kotlin.Int? = null,

    @Json(name = "pageSize")
    val pageSize: kotlin.Int? = null,

    @Json(name = "totalCount")
    val totalCount: kotlin.Int? = null,

    @Json(name = "items")
    val items: kotlin.collections.List<StoredDeviceFilterWithoutTermDto>? = null

)

Despite those fields are not all nullable in the swagger.json.

openapi-generator version

6.3.0

OpenAPI declaration file content or url

swagger.json

Generation Details

java -jar openapi-generator-cli.jar generate ^
     -g kotlin ^
     --library jvm-retrofit2 ^
     -i "https://app-hsm-clse-stage.azurewebsites.net/swagger/standardV1/swagger.json" ^
     -p groupId=com.mycompany ^
     -p artifactId=mycompany-myproduct-client-kotlin ^
     -p artifactVersion=1.0.0 ^
     -p basePackage=com.mycompany.myproduct.networking ^
     -p packageName=com.mycompany.myproduct.networking ^
     -p configPackage=com.mycompany.myproduct.networking.config ^
     -p apiPackage=com.mycompany.myproduct.networking.api ^
     -p modelPackage=com.mycompany.myproduct.networking.model ^
     -p sourceFolder=src/main/java ^
     -p dateLibrary=java8 ^
     -p java8=true ^
     -p useRxJava3=true ^
     
pause

Steps to reproduce

Run above batch file. Check all model classes. For example IDs are never nullable in our swagger.json, the generated code is always nullable, no matter which one.

Related issues/PRs

Suggest a fix

On SO somebody suggested a workaround, but I strongly think this is a bug.

Reproduce the issue with the supplied swagger.json URL and Kotlin jvm-retrofit2 generation command, then inspect the generated model classes shown in the report. Compare their nullable fields with the OpenAPI declaration; done means non-nullable schema fields generate as non-nullable Kotlin properties, with regression coverage added where the generator's existing tests permit.