Quantitative scales shouldn’t treat null as zero.

[Mike Bostock]

const scale = d3.scaleLinear();
scale(null); // returns 0
scale(undefined); // returns undefined

The issue is that quantitative scales coerce to numbers, which is good, but +null is zero, which is bad. We should be consistent with d3-array and treat null as undefined.

[Mike Bostock]

Fixed in #241.