docs: def is_null(value, msg=None, except_=None):
      '''
      ie, "is not defined"
      '''
      # dataframes, even if empty, are not considered null
      value = False if hasattr(value, 'empty') else value
      result = bool(
          value is None or
          value != value or
          repr(value) == 'NaT')
      if except_:
          return is_true(result, msg=msg, except_=except_)
      else:
          return bool(result)