[color-chart] Where should the points corresponding to `null` and `NaN` be plotted?

[DmitrySharabin]

For example:

<color-chart y="oklch.h">
	<color-scale colors="black, white"></color-scale>
</color-chart>

As @LeaVerou pointed out, null corresponds to CSS none; NaN is distinct.

[DmitrySharabin]

(from @LeaVerou) We should plot null as the same as the previous value.

There's another thing to consider, though: What if there are no previous values, like if we plot two colors (black and white) on the oklch.h chart?