Provide optional compatibility with CSS LCH->Lab conversion quirk?

CSS has a rule that when converting from LCh to Lab, if hue is undefined, a and b should be considered zero. https://drafts.csswg.org/css-color-4/#lch-to-lab. Such logic is generally awkward and would require us to break up the generic handling of colors to inject a special, LCh to Lab-specific logic. These aren't even standard rules you see anywhere else, this assumption is CSS-specific. We often provide a way to mimic CSS through additional options; maybe this is another way we might consider adding a conversion option. Unfortunately, it won't be as clean to implement.