Fix SketchMapMonoid zero

[jcoveney]

So, I will add some tests to make sure that this is an issue but I was round-tripping the monoid zero and getting errors, and it looks like this change in the old to new transition is causing them. I'm not sure how we wouldn't run into this in our tests though?! I'll add a failing test but I was hoping @cheecheeo might be able to opine

[cheecheeo]

Looks good to me. I wasn't keeping serialization in mind when I wrote that code :-/. 👍

[cheecheeo]

For serialization purposes this change needs to go in:

It used to be the case that if we serialized the a zero sketchmap (width = 2, depth = 3) we would get this on disk:

0 0 0
0 0 0

With my changes we get this on disk

Z -- where Z is AdaptiveMatrix.monoid.zero ~= SparseVector ~= Map, which is not represented the same way on disk.

When I was writing this code, I was thinking purely of satisfying the monoid laws and thought that AdaptiveMatrix.monoid.zero was simpler than creating the matrix of proper width and height and filling it with V's zeros.