Fow now just don't crash on them, instead give empty result for those
cells.

Change-Id: I9edd231b00fa00af95408a550484da74c98275da
Reviewed-on: https://gerrit.libreoffice.org/43319Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>