[BUG][Python] Object with ref in additionalProperties without property present raises an AttributeError

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

When there is optional property defined that is a map (= object with additionalProperties that points to a ref/enum), then Python will not be able to decode the json data if the property is not present.
It fails with: AttributeError: 'NoneType' object has no attribute 'items'

openapi-generator version

7.9.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Test
  description: Test
  version: 0.1.0
servers:
  - url: "http://localhost:8080"
paths:
  /status:
    get:
      operationId: getStatus
      responses:
        '200':
          description: ok
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Status"
components:
  schemas:
    Status:
      type: object
      properties:
        incorrect:
          type: object
          additionalProperties:
            $ref: "#/components/schemas/State"
    State:
      type: string
      enum: ["disabled","enabled","unknown"]

Generation Details

docker run --rm \
    -v "$(pwd):/local" \
    openapitools/openapi-generator-cli:v7.9.0 generate \
    -i /local/rc23-api.yaml \
    -g python \
    -o /local/out \
    --additional-properties=packageVersion=1.0.0,packageName=my_client,projectName=my-api

Steps to reproduce

Call getStatus where the server omits the "incorrect" property in the json value.
This is interpreted as the property being "None" by Python.

Related issues/PRs

Looks related to #18628.

Suggest a fix

I created a PR (soon) to refer to this bug.

Use the supplied OpenAPI YAML and Docker generation command to reproduce the getStatus response with the optional "incorrect" property omitted. Trace the generated Python client's decoding path for that response; done means the response decodes without the reported AttributeError when the map property is absent.