code: def test_empty_categorical(observed):
      # GH#21334
      cat = Series([1]).astype("category")
      ser = cat[:0]
      gb = ser.groupby(ser, observed=observed)
      result = gb.nunique()
      if observed:
          expected = Series([], index=cat[:0], dtype="int64")
      else:
          expected = Series([0], index=cat, dtype="int64")
      tm.assert_series_equal(result, expected)
  
imports: import builtins
  import datetime
  from string import ascii_lowercase
  import numpy
  import pytest
  from pandas._libs.tslibs import iNaT
  import pandas
  from pandas import DataFrame, MultiIndex, Series, Timestamp, date_range, isna
  import pandas._testing
  from pandas.util import _test_decorators
label_desc: Use image node_id 40 for calling a global function with example usage: test_empty_categorical(observed) without return types