Resolves fdo#84283: Export to PDF with footnotes in tables crash

Don't crash when there's no rectangles

Change-Id: I7e713025cacd449f708176254e05d40c0929577f
Reviewed-on: https://gerrit.libreoffice.org/11634
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-on: https://gerrit.libreoffice.org/11645