When spreadsheet contains more that 84 cells, and these
cells are empty, but contains additional data (border color,
text color, specific formatting), the remaining rows are
ignored during export to .xlsx and .xls
As a result such empty rows are not saved during export.
This patch is fixing most cases and make sure that at least
first 84 empty cells will be preserved.
It is not impacting the performance as it is still notchecking
next columns when the visible cells are more that 84 characters.
This patch improve exporting empty cells, but not impacts
performance.

Change-Id: Ia7027e2c8a2fd48ab4a0e840f970d57167454451
Reviewed-on: https://gerrit.libreoffice.org/29899Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>