Print fails on None

[otsaloma]

  File "test.py", line 14, in test
    print(res.head())
  File "/usr/local/lib/python3.8/dist-packages/dataiter/data_frame.py", line 160, in __str__
    return self.to_string()
  File "/usr/local/lib/python3.8/dist-packages/dataiter/data_frame.py", line 740, in to_string
    columns = {colname: util.pad(
  File "/usr/local/lib/python3.8/dist-packages/dataiter/data_frame.py", line 740, in <dictcomp>
    columns = {colname: util.pad(
  File "/usr/local/lib/python3.8/dist-packages/dataiter/deco.py", line 30, in wrapper
    return list(value)
  File "/usr/local/lib/python3.8/dist-packages/dataiter/util.py", line 67, in pad
    width = max(len(x) for x in strings)
  File "/usr/local/lib/python3.8/dist-packages/dataiter/util.py", line 67, in <genexpr>
    width = max(len(x) for x in strings)
TypeError: object of type 'NoneType' has no len()