[REVIEW] fix for fdo#46332, crash while opening xls file

[Markus Mohrhard]

Hey,

this patch ([1]) fixes the crash while opening the test document. The
file contains 5 empty categories while we assume that all categories
are non empty which results in a crash in
chart2/source/tools/ExplicitCategoriesProvider.cxx:364

This patch just skips all empty categories. I checked the file and it
looks exactly the same in Excel and I have no idea why the file
contains 5 empty categories.

I think that this patch is safe and simple and we should push it to 3-5.

Regards,
Markus

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=e4f14d089d81459ac2bf3c40e75e6cfd348b58c5