code: def test_struct_with_null_memory_usage():
      df = cudf.DataFrame(
          {
              "a": cudf.Series([1, 2, -1, -1, 3], dtype="int64"),
              "b": cudf.Series([10, 20, -1, -1, 30], dtype="int64"),
          }
      )
      s = df.to_struct()
      assert s.memory_usage() == 80
  
      s[2:4] = None
      assert s.memory_usage() == 272
  
imports: import numpy
  import pandas
  import pyarrow
  import pytest
  import cudf
  from cudf.core.dtypes import StructDtype
  from cudf.testing._utils import DATETIME_TYPES, TIMEDELTA_TYPES, assert_eq
label_desc: Use image node_id 23 for calling a global function with example usage: test_struct_with_null_memory_usage() without return types