code: def test_null_is_null_for_dtype(
      sort, dtype, nulls_fixture, nulls_fixture2, test_series
  ):
      # GH#48506 - groups should always result in using the null for the dtype
      df = pd.DataFrame({"a": [1, 2]})
      groups = pd.Series([nulls_fixture, nulls_fixture2], dtype=dtype)
      obj = df["a"] if test_series else df
      gb = obj.groupby(groups, dropna=False, sort=sort)
      result = gb.sum()
      index = pd.Index([na_value_for_dtype(groups.dtype)])
      expected = pd.DataFrame({"a": [3]}, index=index)
      if test_series:
          tm.assert_series_equal(result, expected["a"])
      else:
          tm.assert_frame_equal(result, expected)
  
imports: import numpy
  import pytest
  from pandas.compat.pyarrow import pa_version_under10p1
  from pandas.core.dtypes.missing import na_value_for_dtype
  import pandas
  import pandas._testing
  from pandas.tests.groupby import get_groupby_method_args
label_desc: Use image node_id 15 for calling a global function with example usage: test_null_is_null_for_dtype(sort, dtype, nulls_fixture, nulls_fixture2, test_series) without return types