This second related situation is about when there is drawingML fallback
for the SmartArt, but it's ampty shape tree, which is never created with
MSO2010+ (those shape trees always have at least one element). This is
as bad as the missing drawingML fallback, so warn on it.

Change-Id: I539d05154a4d1fcd5871dfc29616f77eb9945454
Reviewed-on: https://gerrit.libreoffice.org/34131Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>