code: def test_write_parquet_with_null_col(tmp_path: Path) -> None:
      tmp_path.mkdir(exist_ok=True)
  
      df1 = pl.DataFrame({"nulls": [None] * 2, "ints": [1] * 2})
      df2 = pl.DataFrame({"nulls": [None] * 2, "ints": [1] * 2})
      df3 = pl.DataFrame({"nulls": [None] * 3, "ints": [1] * 3})
      df = df1.vstack(df2)
      df = df.vstack(df3)
      file_path = tmp_path / "with_null.parquet"
      df.write_parquet(file_path, row_group_size=3)
      out = pl.read_parquet(file_path)
      assert_frame_equal(out, df)
  
imports: from __future__ import annotations
  import io
  from datetime import datetime, time, timezone
  from decimal import Decimal
  from typing import TYPE_CHECKING
  import numpy
  import pandas
  import pyarrow
  import pyarrow.dataset
  import pyarrow.parquet
  import pytest
  import polars
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 37 for calling a global function with example usage: test_write_parquet_with_null_col(tmp_path) without return types