(rbd2vhd.py) Create an empty vhd if rbd diff doesn't contain data.
(rbd2vhd.py) Create an empty vhd if rbd diff doesn't contain data.

For example when we try to export difference between image and snapshot
when there weren't any changes in image after snapshot.
Previously an error was raised in such case.
suzj801/RBDSR,rposudnevskiy/RBDSR,rposudnevskiy/RBDSR,suzj801/RBDSR