code: def get_null_constants(df):
      null_values = {}
      for col, dtype in df.dtypes.to_dict().items():
          if np.issubdtype(dtype, np.datetime64):
              null_values[col] = np.datetime64("nat")
          elif np.issubdtype(dtype, np.number):
              null_values[col] = np.nan
      return null_values
  
imports: from blazingsql import BlazingContext
  from blazingsql import DataType, S3EncryptionType
  import numpy
  import pandas
  import os
  import subprocess
  import sys
  from EndToEndTests import tpchQueries
label_desc: Use image node_id 11 for calling a global function with example usage: get_null_constants(df) and returns: null_values