code: def test_dti_tz_nat(self, tzstr):
      idx = DatetimeIndex([Timestamp("2013-1-1", tz=tzstr), pd.NaT])
  
      assert isna(idx[1])
      assert idx[0].tzinfo is not None
  
imports: from datetime import datetime, timedelta, timezone, tzinfo
  from dateutil.tz import gettz
  import numpy
  import pytest
  import pytz
  from pandas._libs.tslibs import conversion, timezones
  import pandas
  from pandas import DatetimeIndex, Timestamp, bdate_range, date_range, isna, to_datetime
  import pandas._testing
label_desc: Use image node_id 8 for calling the TestDatetimeIndexTimezones obj's underlying member method code with example usage: obj.test_dti_tz_nat(tzstr) without return types