code: def test_hetero_graph_none(self):
      G = generate_simple_hete_graph(add_edge_type=False)
      hete = HeteroGraph(G)
      message_types = hete.message_types
      for message_type in message_types:
          self.assertEqual(message_type[1], None)
  
file_path: deepsnap/tests/test_hetero_graph.py
imports: import math
  import unittest
  from torch.utils.data import DataLoader
  from tests.utils import generate_simple_hete_graph, generate_dense_hete_graph, generate_dense_hete_multigraph
  from deepsnap.batch import Batch
  from deepsnap.hetero_graph import HeteroGraph
label_desc: Use image node_id 5 for calling the TestHeteroGraph obj's underlying member method code with example usage: obj.test_hetero_graph_none() without return types