code: def test_mixed_str_null(nulls_fixture):
      values = np.array(["b", nulls_fixture, "a", "b"], dtype=object)
      result = safe_sort(values)
      expected = np.array(["a", "b", "b", nulls_fixture], dtype=object)
      tm.assert_numpy_array_equal(result, expected)
  
imports: from collections import defaultdict
  from datetime import datetime
  from itertools import product
  import numpy
  import pytest
  from pandas import NA, DataFrame, MultiIndex, Series, array, concat, merge
  import pandas._testing
  from pandas.core.algorithms import safe_sort
  import pandas.core.common
  from pandas.core.sorting import _decons_group_index, get_group_index, is_int64_overflow_possible, lexsort_indexer, nargsort
label_desc: Use image node_id 3 for calling a global function with example usage: test_mixed_str_null(nulls_fixture) without return types