Avoid to iterate over empty list in DictMismatch

It is a little optimization of describe function. If there wasn't found any
differences, there is loop iterating over empty list. It isn't necessary,
because condition a few lines above do the same.

Change-Id: I65067801281c8703f7fa4d1430dcc578d28c3542
Merge "Avoid to iterate over empty list in DictMismatch"