code: def insert_explicit_nulls(unischema, row_dict):
      """If input dictionary has missing fields that are nullable, this function will add the missing keys with
      None value.
  
      If the fields that are missing are not nullable, a ``ValueError`` is raised.
  
      :param unischema: An instance of a unischema
      :param row_dict: dictionary that would be checked for missing nullable fields. The dictionary is modified inplace.
      :return: None
      """
      for field_name, value in unischema.fields.items():
          if field_name not in row_dict:
              if value.nullable:
                  row_dict[field_name] = None
              else:
                  raise ValueError(
                      "Field {} is not found in the row_dict, but is not nullable.".format(
                          field_name
                      )
                  )
  
imports: import copy
  import re
  import sys
  import warnings
  from collections import namedtuple, OrderedDict
  from decimal import Decimal
  from typing import Dict, Any, Tuple, Optional, NamedTuple
  import numpy
  import pyarrow
  import six
  from pyarrow.lib import ListType
  from pyarrow.lib import StructType
  from six import string_types
label_desc: Use image node_id 6 for calling a global function with example usage: insert_explicit_nulls(unischema, row_dict) without return types