code: def _column_pad_filter(self, next_filter):
          
          next(next_filter)
          while True:
              line = list((yield))
              for i, col in enumerate(line):
                  if col is None:
                      line[i] = self._get_blank_cell(i)
              next_filter.send(line)
docs: Expand blank lines caused from overflow of other columns to blank
          whitespace.  E.g.
              INPUT: [
                  ["1a", "2a"],
                  [None, "2b"],
                  ["1b", "2c"],
                  [None, "2d"]
              ]
              OUTPUT: [
                  ["1a", "2a"],
                  [<blank>, "2b"],
                  ["1b", "2c"],
                  [<blank>, "2d"]
              ]