code: def test_unique_null() -> None:
      s0 = pl.Series([])
      assert_series_equal(s0.unique(), s0)
  
      s1 = pl.Series([None])
      assert_series_equal(s1.unique(), s1)
  
      s2 = pl.Series([None, None])
      assert_series_equal(s2.unique(), s1)
  
file_path: polars/py-polars/tests/unit/operations/unique/test_unique.py
imports: from __future__ import annotations
  from datetime import date
  import pytest
  import polars
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 9 for calling a global function with example usage: test_unique_null() without return types