code: def test_combines_two_empty(self):
      accumulator1 = variance_util.MeanCovAccumulator()
      accumulator2 = variance_util.MeanCovAccumulator()
      accumulator1.merge(accumulator2)
  
      self.assertIsNone(accumulator1.mean)
      self.assertIsNone(accumulator1.covariance)
  
imports: from absl.testing import absltest
  from absl.testing import parameterized
  import numpy
  from tensorflow_data_validation.utils import variance_util
label_desc: Use image node_id 7 for calling the MeanCovAccumulatorTest obj's underlying member method code with example usage: obj.test_combines_two_empty() without return types