code: def test_empty_graph(self):
      G1 = nx.Graph()
      G2 = nx.Graph()
      gparams = _GraphParameters(G1, G2, None, None, None, None, None)
      assert len(set(_matching_order(gparams))) == 0
  
imports: import itertools
  import pytest
  import networkx
  from networkx import vf2pp_is_isomorphic, vf2pp_isomorphism
  from networkx.algorithms.isomorphism.vf2pp import _consistent_PT, _cut_PT, _feasibility, _find_candidates, _find_candidates_Di, _GraphParameters, _initialize_parameters, _matching_order, _restore_Tinout, _restore_Tinout_Di, _StateParameters, _update_Tinout
label_desc: Use image node_id 1 for calling the TestNodeOrdering obj's underlying member method code with example usage: obj.test_empty_graph() without return types