Check additionally for None when checking for single cells

[fzumstein]

Otherwise, something like [None, None] causes an Error when written to Excel.

Correct Line 472 and possibly other ocurrencies (TODO: factor out the check for single cells).

# Simple Lists: Turn into list of lists
if isinstance(data, list) and (isinstance(data[0], (numbers.Number, string_types, time_types) or data[0] is None):