fdo#69289 RTF import: handle cells with zero width

Change-Id: I31b12a5afa5d501d57b56f515ee1d2fd46a09b32
(cherry picked from commit 3e2cda224e4119b31d85263ff16a383e693dcbbd)
Reviewed-on: https://gerrit.libreoffice.org/8585
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>