Test for empty keys instead of missing key's value

The generated bottom dictionary lattice has no keys. That's what makes it
bottom. An equally valid bottom would be a hash which maps all its elements to
bottom. I've add two tests cases that ensure the generated bottom is equivalent
to bottoms which maps keys.
The default value for a dictionary lattice is the domain's bottom

I mistakenly put the top element as the default in the lte case. I probably was
thinking there was a duality.