[REQ] [python-flask] Distinction between "null" and "undefined"

Is your feature request related to a problem? Please describe.

It does not seems to be possible to distinct "null" and "undefined" when receiving request body parameters in Python.

For example :
Schema :

User:
  type: object
  properties:
    firstname:
      type: string
    lastname:
      type: string
    email:
      type: string
      nullable: true
  required:
    - firstname
    - lastname

Payload A :

{
  firstname: "John",
  lastname: "Doe",
  email: null,
}

Payload B :

{
  firstname: "John",
  lastname: "Doe",
}

Using payload A and B do not have the same meaning using Json Merge Patch format from RFC 7386.
(See : https://datatracker.ietf.org/doc/html/rfc7386)

Payload A should set 'None' to attribute email and Payload B should let the email untouched.

Describe the solution you'd like

It could be nice to have a solution like JsonNullable used on the Java Codegen (See : https://github.com/OpenAPITools/jackson-databind-nullable)
We could wrap with "nullable" attributes from Python model.mustache and it will allow to distinct between "None" and "undefined" using ".is_set" or ".get".

Note :
If a "to_dict" method is present on the model, it should not generate Entry<Key,Value> for the "undefined" attributes.

Start by inspecting the Python Flask model.mustache template and the generated model behavior for nullable fields. Compare the requested distinction with Java's JsonNullable approach, including the mentioned is_set/get semantics. Done means payloads with null and omitted fields remain distinguishable, and to_dict omits undefined attributes.