code: def test_all_pairs_lca_null_graph(self):
      pytest.raises(
          nx.NetworkXPointlessConcept, all_pairs_lca, nx.DiGraph()
      )
  
imports: from itertools import chain, combinations, product
  import pytest
  import networkx
label_desc: Use image node_id 9 for calling the TestDAGLCA obj's underlying member method code with example usage: obj.test_all_pairs_lca_null_graph() without return types