don't warn for empty VBA structure, fdo#61059

Change-Id: I0d0f7fa216fcea7ca893120ad183dc3758ca51d4
(cherry picked from commit 2e803712c4c2b844dab1fefdfbda12058a5f1ab7)
Reviewed-on: https://gerrit.libreoffice.org/6573
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>