docs: def fillna(self, value):
          """Returns Index with missing values replaced with value.
  
          Parameters
          ----------
          value : {int, float, bytes, bool}
              Scalar value to replace missing values with.
  
          Returns
          -------
          Index
              With missing values replaced.
  
          """
          if not is_scalar(value):
              raise TypeError('Value to replace with is not a valid scalar')
  
          return Index(weld_replace(self.weld_expr,
                                    self.weld_type,
                                    default_missing_data_literal(self.weld_type),
                                    value),
                       self.dtype,
                       self.name)