[BUG][Kotlin] required yet nullable schema properties are not optional in generated models

[by-zhou]

Description

Some schemas have required but nullable properties in our open API specification. These are mapped to non-optional in the generated models.

openapi-generator version

4.2.0

OpenAPI declaration file content or url

components:
  schemas:
Foo:
      type: object
      required: [name]
      nullable: false
      properties:
        name:
          type: string
          nullable: true
...

Command line used for generation

Steps to reproduce

Related issues/PRs

Similar issue in Java: #4530

Suggest a fix/enhancement

Not sure if I miss something else. I think the only thing missing is a question mark at the very end of
the template "data_class_req_var.mustache".

...{{#isNullable}}?{{/isNullable}}

[AlexanderEggers]

@pychoude I have created a PR based on your suggestion. Let me know what you think.