docstr: Should return None if the structure model is empty
docstr_tok: ["Should","return","None","if","the","structure","model","is","empty"]
code: def test_empty(self):
      """Should return None if the structure model is empty"""
  
      sm = StructureModel()
      assert sm.get_largest_subgraph() is None
  
imports: import pytest
  from networkx.exception import NodeNotFound
  from causalnex.structure import StructureModel
label_desc: Use image node_id 3 for calling the TestStructureModelGetLargestSubgraph obj's underlying member method code with example usage: obj.test_empty() without return types