[BUG][KOTLIN-SPRING] Fields marked as required: false and nullable: false are serialised as null in request body

Description

In the generated Kotlin models, fields marked as required: false and nullable: false are mapped to Type? = null, which is fine. However, because of this, the field ends up being included in the JSON request body as "field": null, even though the OpenAPI schema explicitly states that null is not allowed.

This leads to invalid requests being sent to APIs that correctly reject null values for non-nullable fields. Fields that are not required and not nullable should simply be omitted from the request body if they are null.

openapi-generator version

gradle plugin: org.openapi.generator 7.12.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Example API
  version: 1.0.0
  description: Simple API to demonstrate issue with nullable and optional fields in Kotlin code generation

paths:
  /examples:
    post:
      summary: Create an example resource
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/ExampleDto'
      responses:
        '200':
          description: Successful creation

components:
  schemas:
    ExampleDto:
      type: object
      properties:
        name:
          type: string
          description: "A name field that is optional but not nullable"
          nullable: false
      required: []

Suggest a fix

Consider adding @JsonInclude(JsonInclude.Include.NON_NULL) at the class level to omit null fields by default.
Then, for fields where required: true or nullable: true, use @JsonInclude(JsonInclude.Include.ALWAYS) to ensure null values are included when needed

Use the supplied OpenAPI 3.0.3 declaration with the org.openapi.generator 7.12.0 Gradle plugin to reproduce the generated Kotlin model and request-body serialization. Inspect how optional, non-nullable fields are represented and serialized; done means a null field is omitted while the required or nullable cases described in the issue retain their intended behavior.