'Tests that the null graph has empty node boundaries.'

def test_null_graph(self):

    null = nx.null_graph()
    assert_equal(nx.node_boundary(null, []), set())
    assert_equal(nx.node_boundary(null, [], []), set())
    assert_equal(nx.node_boundary(null, [1, 2, 3]), set())
    assert_equal(nx.node_boundary(null, [1, 2, 3], [4, 5, 6]), set())
    assert_equal(nx.node_boundary(null, [1, 2, 3], [3, 4, 5]), set())
