code: def test_n_unique_null() -> None:
      assert pl.Series([]).n_unique() == 0
      assert pl.Series([None]).n_unique() == 1
      assert pl.Series([None, None]).n_unique() == 1
  
imports: from __future__ import annotations
  import pytest
  import polars
label_desc: Use image node_id 3 for calling a global function with example usage: test_n_unique_null() without return types