Add class around diff-empty and add it as notice

Add the moment the message is added within a class=diff-multi,
which is not true.

Bug: 53168
Follow-Up: I458fb688b0001fb674ece65b3fdabf56fc658a29
Change-Id: Ic3040ceca4ff1459181c84f041490e9e72b12802
Merge "Add class around diff-empty and add it as notice"