Averaging when all colors are transparent

Currently, averaging has a simple rule where, when a color is transparent, its color components do not contribute to the average, as if the channel is missing. If all colors are transparent, you end up with color channels as undefined, which makes sense because none of the color components can contribute to the average. The question is whether this is preferable to the user. Should we just normalize the undefined value to whatever is neutral for the color space (usually zero)? So instead of getting `color(srgb none none none/ 0)` as a result, you would get `color(srgb 0 0 0 / 0)`.