code: def test_NA_string_dtype():
      df = cudf.DataFrame({"A": ["a", "b", "cdef", "", "g"]})
      df["B"] = df["A"].astype("object")
      df.at[1, "B"] = cudf.NA  # Set one item to null
  
      # Test for correctness and null handling:
      col = df.__dataframe__().get_column_by_name("B")
      assert col.dtype[0] == _DtypeKind.STRING
      assert col.null_count == 1
      assert col.describe_null == (3, 0)
      assert col.num_chunks() == 1
      assert_from_dataframe_equals(df, allow_copy=False)
      assert_from_dataframe_equals(df, allow_copy=True)
  
imports: from typing import Any, Tuple
  import cupy
  import pandas
  import pytest
  import cudf
  from cudf.core._compat import PANDAS_GE_150
  from cudf.core.buffer import as_buffer
  from cudf.core.column import as_column, build_column
  from cudf.core.df_protocol import DataFrameObject, _CuDFBuffer, _CuDFColumn, _DtypeKind, _MaskKind, _protocol_buffer_to_cudf_buffer, from_dataframe, protocol_dtype_to_cupy_dtype
  from cudf.testing._utils import assert_eq
label_desc: Use image node_id 20 for calling a global function with example usage: test_NA_string_dtype() without return types