code: def test_preprocessing_pipeline(direction: StudyDirection) -> None:
      p1 = _preprocessing.NullPreprocessing()
      p2 = _preprocessing.NullPreprocessing()
  
      pipeline = _preprocessing.PreprocessingPipeline([p1, p2])
  
      with mock.patch.object(p1, "apply") as a1:
          with mock.patch.object(p2, "apply") as a2:
              pipeline.apply([], direction)
  
              a1.assert_called_once()
              a2.assert_called_once()
  
file_path: optuna/tests/terminator_tests/improvement_tests/test_preprocessing.py
imports: from __future__ import annotations
  from unittest import mock
  import numpy
  import pytest
  from optuna.distributions import CategoricalDistribution
  from optuna.distributions import FloatDistribution
  from optuna.distributions import IntDistribution
  from optuna.study._study_direction import StudyDirection
  from optuna.terminator.improvement import _preprocessing
  from optuna.trial import create_trial
  from optuna.trial import FrozenTrial
label_desc: Use image node_id 2 for calling a global function with example usage: test_preprocessing_pipeline(direction) without return types