I would introduce a result line for this:

Collection result = mm.values

I also would name this method:

testValues_whenEmptyCollection

This makes it a lot easier to find the method covering the values functionality. So please try to use the naming convention. If things are done in the same way, I can understand your code a lot easier, but you can also understand my code a lot easier. This means we have more time doing actual work and don't need to waste time on things that are just a bit different. 

Imagine how all those Europeans feel when do go to e.g. the UK where they drive on the other side of the road. It is a minor choice, but makes life more complicated than needs to be.