[BUG][scala-sttp4] Optional model fields serialize as JSON null instead of being omitted (regression vs scala-sttp)

[vivekmahajan]

Description

In the scala-sttp4 generator with jsonLibrary=circe, optional model fields set to None are serialized as JSON null instead of being omitted. This is a regression relative to the scala-sttp (sttp3) generator, whose generated encoders explicitly drop absent optional fields.

Generated Pet.scala (sttp4, circe):

implicit val encoder: Encoder[Pet] = deriveEncoder   // Pet("rex", None) => {"name":"rex","tag":null}

Same schema through the scala-sttp generator:

implicit val encoderPet: Encoder[Pet] = Encoder.instance { t =>
  Json.fromFields{
    Seq(
      Some("name" -> t.name.asJson),
      t.tag.map(v => "tag" -> v.asJson)   // None => field omitted
    ).flatten
  }
}

For OpenAPI, an optional non-nullable property must be absent when unset — many servers (e.g. anything with strict validation, FastAPI/pydantic non-nullable fields with defaults) reject an explicit null with a 422. In practice every request-body PUT/POST from a generated sttp4 client that leaves an optional field at its None default can be rejected, even though the same call from an sttp3-generated client works.

The json4s branch is unaffected (json4s omits None by default), as is scala-sttp4-jsoniter (transientNone defaults to true).

openapi-generator version

7.23.0 (also reproduces on current master).

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Optional field test
  version: 1.0.0
paths:
  /pets:
    post:
      operationId: createPet
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Pet'
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Pet'
components:
  schemas:
    Pet:
      type: object
      required:
        - name
      properties:
        name:
          type: string
        tag:
          type: string

Generation Details

java -jar openapi-generator-cli.jar generate -g scala-sttp4 -i pet.yaml -o out --additional-properties=jsonLibrary=circe

Steps to reproduce

1. Generate a client from the spec above.
2. Pet(name = "rex", tag = None).asJson.noSpaces → {"name":"rex","tag":null}.
3. Generate the same spec with -g scala-sttp → the encoder omits tag when None.

Suggest a fix

PR incoming: append .mapJson(_.dropNullValues) to the generated circe encoders in the scala-sttp4 templates (plain models and oneOf/sealed-trait encoders), restoring sttp3 parity: None ⇒ field omitted.