code: def test_astype_to_str_preserves_na(self, value, string_value):
      # https://github.com/pandas-dev/pandas/issues/36904
      ser = Series(["a", "b", value], dtype=object)
      result = ser.astype(str)
      expected = Series(["a", "b", string_value], dtype=object)
      tm.assert_series_equal(result, expected)
  
imports: from datetime import datetime, timedelta
  from importlib import reload
  import string
  import sys
  import numpy
  import pytest
  from pandas._libs.tslibs import iNaT
  import pandas.util._test_decorators
  from pandas import NA, Categorical, CategoricalDtype, DatetimeTZDtype, Index, Interval, NaT, Series, Timedelta, Timestamp, cut, date_range, to_datetime
  import pandas._testing
label_desc: Use image node_id 18 for calling the TestAstype obj's underlying member method code with example usage: obj.test_astype_to_str_preserves_na(value, string_value) without return types