We already warn when the SmartArt doesn't have a drawingML fallback, but
having a reference to a fallback that is not a valid one (so resolves to
an empty fragment path) is also unusable. So warn in that case as well.

(cherry picked from commit 2d732b49)

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.

(cherry picked from commit 8c6776f2)

Change-Id: I03b2325003b4eb38b1347a88899799c6d5c33606
Reviewed-on: https://gerrit.libreoffice.org/34196Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>