code: def impute_null(X, impute_val=None):
      """
      Impute null values. Null values are
      determined using `pd.isnull`.
  
      Args:
          X (numpy array): input data
          impute_val (object): value to impute with
      Returns:
          X (numpy array)
      """
      if impute_val is None:
          return X
      else:
          X[pd.isnull(X)] = impute_val
          return X
  
imports: import warnings
  import pandas
  import numpy
  from scipy import sparse
  from sklearn.base import BaseEstimator, TransformerMixin
  from sklearn.preprocessing import MultiLabelBinarizer, FunctionTransformer, LabelEncoder, normalize
  from sklearn.feature_extraction.text import HashingVectorizer
  from sklearn import feature_selection
label_desc: Use image node_id 5 for calling a global function with example usage: impute_null(X, impute_val) and returns: X, X