[Delta backup] Merge should not fail when delta contains no data

[julien-f]

It can happen if no data has changed on the disk between both exports.

@olivierlambert Can you test this?

[olivierlambert]

How could I test this?

[julien-f]

Delta backup, retention set to 1, backup twice the VM without running it should do the trick.

[olivierlambert]

It works perfectly. The only "glitch" I have: the merge information aren't displayed. Maybe it's normal if there is no merge in the end.

But anyway, no errors.