HWB (and HSL) with undefined Hue

[svgeesus]

In the original Alvey Ray Smith paper defining HWB is the comment

H is returned UNDEFINED if W == 1 – B

Achromatic colors currently return odd hue angles; black has hue 0 (a red) while white has hue 137.281(a cyanish green).

[LeaVerou]

This generally applies to achromatic colors in every polar color space/model. I think we should bite the bullet and use NaN which would also interpolate nicely. It seems that both chroma.js and d3-color do that.