code: def data():
      """Generates test dataframe for null imputation"""
      return pd.DataFrame(
          {
              "a": (0.0, np.nan, -1.0, 1.0),
              "b": (np.nan, 2.0, np.nan, np.nan),
              "c": (2.0, 3.0, np.nan, 9.0),
              "d": (np.nan, 4.0, -4.0, 16.0),
          }
      )
  
file_path: greykite/greykite/tests/sklearn/transform/test_null_transformer.py
imports: import numpy
  import pandas
  import pytest
  from sklearn.exceptions import NotFittedError
  from testfixtures import LogCapture
  from greykite.common.constants import LOGGER_NAME
  from greykite.common.python_utils import assert_equal
  from greykite.sklearn.transform.null_transformer import NullTransformer
label_desc: Use image node_id 1 for calling a global function with example usage: data() and returns: pd