[BUG][PYTHON] ComposedSchema - Type null not handled properly in toExampleValue

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

PythonClientCodegen when run against openapi.yaml will produce the followin warning output.

[main] WARN  o.o.c.l.AbstractPythonCodegen - Type null not handled properly in toExampleValue

openapi-generator version

5.2.1

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: API
  version: 1.0.0
paths:
  /pet:
    get:
      operationId: get_handler
      parameters:
      - in: query
        name: pet
        schema:
          $ref: '#/components/schemas/Dog'
      responses:
        '200':
          content:
            application/json:
              schema:
                type: object
          description: Success
components:
  schemas:
    Pet:
      type: object
      properties:
        pet_type:
          type: string
    Dog:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          properties:
            bark:
              type: boolean

Generation Details

openapi-generator generate -i openapi.yaml -g python-flask -t templates --package-name 

Steps to reproduce

Generate the client using the command above

Related issues/PRs

No similar PR

Suggest a fix

Handle the ComposedSchema example generation in AbstractPythonCodegen -> toExampleValueRecursive.

Start in AbstractPythonCodegen.toExampleValueRecursive, using the provided openapi.yaml and the Python Flask generation command to reproduce the warning. The work is complete when the ComposedSchema example generation handles the null type without emitting the reported warning.