[BUG][Kotlin] Nullable attribute is ignored if field is not required

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

When generating model with fields not required nullable attribute is ignored

openapi-generator version

5.3.0

OpenAPI declaration file content or url

components:
  schemas:
    Cat:
          type: object
          xml:
            name: cat
          properties:
            id:
              type: integer
              format: int64
              nullable: false
            name:
              type: boolean
              nullable: true

Generation Details

Steps to reproduce

Generate this class. Expected result:

data class Cat (

    @SerializedName("id")
    val id: kotlin.Long,

    @SerializedName("name")
    val name: kotlin.Boolean? = null

)

actual:

data class Cat (

    @SerializedName("id")
    val id: kotlin.Long? = null,

    @SerializedName("name")
    val name: kotlin.Boolean? = null

)

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/4530

Suggest a fix

Copy the solution from /data_class_req_var.mustache to data_class_opt_var.mustache

{{#isNullable}}?{{/isNullable}}{{#defaultValue}} = {{{.}}}{{/defaultValue}}

Start with data_class_req_var.mustache and data_class_opt_var.mustache, then generate the Kotlin model from the OpenAPI schema in the issue. The optional nullable field should preserve its nullable marker while the non-required id remains nullable as shown in the expected output.