code: def test_null_subgraph(self):
      # Subgraph of a null graph is a null graph
      nullgraph = nx.null_graph()
      G = nx.null_graph()
      H = G.subgraph([])
      assert nx.is_isomorphic(H, nullgraph)
  
imports: import pytest
  import networkx
  from networkx import convert_node_labels_to_integers
  from networkx.utils import edges_equal, nodes_equal
label_desc: Use image node_id 31 for calling the HistoricalTests obj's underlying member method code with example usage: obj.test_null_subgraph() without return types