Created by: higs4281
i'm using csvkit to process emailed excel files, but i keep encountering xlsx files with null rows, and in2csv output bails when the null is encountered.
this patch is saving me grief, so i thought i'd offer it. i tried writing a test for it but got deep in weeds trying to replicate the bad xlsx content.
i haven't hit this issue (or been able to replicate it) with the older xls format.