code: def test_prefill_form_null_extra():
      mock_form = mock.Mock()
      mock_form.data = {
          "conn_id": "test",
          "extra": None,
          "conn_type": "test",
      }
  
      cmv = ConnectionModelView()
      cmv._iter_extra_field_names_and_sensitivity = mock.Mock(
          return_value=()
      )
      cmv.prefill_form(form=mock_form, pk=1)
  
imports: from __future__ import annotations
  import json
  from typing import Any
  from unittest import mock
  from unittest.mock import PropertyMock
  import pytest
  from airflow.models import Connection
  from airflow.utils.session import create_session
  from airflow.www.views import ConnectionFormWidget, ConnectionModelView
  from tests.test_utils.www import _check_last_log, _check_last_log_masked_connection, check_content_in_response
label_desc: Use image node_id 8 for calling a global function with example usage: test_prefill_form_null_extra() without return types