code: def test_datetimeindex_union_join_empty(self, sort):
      dti = date_range(start="1/1/2001", end="2/1/2001", freq="D")
      empty = Index([])
  
      result = dti.union(empty, sort=sort)
      expected = dti.astype("O")
      tm.assert_index_equal(result, expected)
  
      result = dti.join(empty)
      assert isinstance(result, DatetimeIndex)
      tm.assert_index_equal(result, dti)
  
file_path: pandas/pandas/tests/indexes/datetimes/test_join.py
imports: from datetime import datetime, timezone
  import numpy
  import pytest
  from pandas import DataFrame, DatetimeIndex, Index, Timestamp, date_range, period_range, to_datetime
  import pandas._testing
  from pandas.tseries.offsets import BDay, BMonthEnd
label_desc: Use image node_id 6 for calling the TestJoin obj's underlying member method code with example usage: obj.test_datetimeindex_union_join_empty(sort) without return types