MapMonoid culls empty values

This just adds a failing test. This was an optimization (and not an unreasonable one), though it changes how users should think about the output of MapMonoid. It does not seem expected that the result of Monoid.plus could result in the key NOT being available, which is exactly what is now possible.

I feel like we should remove this, and add the ability to cull zeros for MapMonoid is so desired. Open to other suggestions.