Let `empty` target cells with no data

The documentation states that the optional property `empty` of the `legendColors` sets the color for dates with value == 0. This is confusing because the documentation also states that the property name `empty` of the `subDomainTitleFormat` targets dates without a value (so value == null).

So does `empty` target value == 0 or value == null?

I would encourage the latter option. In the case of the first option, there is a value which can be colored with the default legend and it makes more sense to do so. Furthermore, there is currently no way to color dates where value == null, which happens to be what I'm currently trying to accomplish.

I updated the code, but I get some failing unit tests. (I also get these before my modifications, so I assume I can ignore them).