If desc is empty we should not try to eval it,
otherwise it would lead to exceptio. So test
for its size as well.

travis-ci: success for zdtm.py: Fix eval error if empty desc file provided
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>