code: def test_map_batches_preserve_empty_blocks(ray_start_regular_shared):
      ds = ray.data.range(10, parallelism=10)
      ds = ds.map_batches(lambda x: [])
      ds = ds.map_batches(lambda x: x)
      assert ds.num_blocks() == 10, ds
  
imports: import itertools
  import math
  import os
  import signal
  import threading
  import time
  from typing import Iterator
  import numpy
  import pandas
  import pyarrow
  import pyarrow.parquet
  import pytest
  import ray
  from ray._private.test_utils import wait_for_condition
  from ray.data.block import BlockAccessor
  from ray.data.context import DataContext
  from ray.data.tests.conftest import *
  from ray.data.tests.util import column_udf, column_udf_class, extract_values
  from ray.tests.conftest import *
label_desc: Use image node_id 20 for calling a global function with example usage: test_map_batches_preserve_empty_blocks(ray_start_regular_shared) without return types