code: def testNGramsEmpty(self):
      with tf.compat.v1.Graph().as_default():
          output_tensor = mappers.ngrams(
              tf.compat.v1.strings.split(tf.constant([""])), (1, 5), ""
          )
          with tf.compat.v1.Session():
              output = output_tensor.eval()
              self.assertEqual((0, 2), output.indices.shape)
              self.assertAllEqual([1, 0], output.dense_shape)
              self.assertEqual(0, len(output.values))
  
imports: import numpy
  import tensorflow
  from tensorflow_transform import mappers
  from tensorflow_transform import test_case
label_desc: Use image node_id 4 for calling the MappersTest obj's underlying member method code with example usage: obj.testNGramsEmpty() without return types