code: def to_nullable_string(value):
          
          if value == None:
              return None
          if type(value) == datetime.date:
              return value.isoformat()
          if type(value) == datetime.datetime:
              if value.tzinfo == None:
                  return value.isoformat() + "Z"
              else:
                  return value.isoformat()
  
          if type(value) == list:
              builder = ''
              for element in value:
                  if len(builder) > 0:
                      builder = builder + ","
                  builder = builder + element
              return builder.__str__()
          return str(value)
docs: Converts value into string or returns None when value is None.
  
          :param value: the value to convert.
  
          :return: string value or None when value is None.