code: def test_no_op(self):
      expected = self.df
      result = msno.nullity_sort(self.df, sort=None)
  
      assert result.equals(expected)
  
imports: import unittest
  import pandas
  import numpy
  import sys
  import missingno
label_desc: Use image node_id 2 for calling the TestNullitySort obj's underlying member method code with example usage: obj.test_no_op() without return types