diff: Display "(No difference)" instead of an empty diff

This makes it easier to see the fact, because without a hint, the user
can mean, that there is something missing, but here the missing is okay
and for that, the message is helpful.

Change-Id: I458fb688b0001fb674ece65b3fdabf56fc658a29
Merge "diff: Display "(No difference)" instead of an empty diff"