Consider empty rings empty slots

[brollb]

This is the fix I alluded to in https://github.com/jmoenig/Snap/issues/2836. If empty rings are not considered empty slots by design, then this can be rejected, of course :) I was going to wait to open the PR but thought it might actually help to explain my thinking in the comments in the issue!

[jmoenig]

I remember changing this for v7, now empty rings inside HOFs like MAP etc. should be considered empty slots. Geez, I don't remember how I did that, though. But I'm hoping that your suggestion now is the official behavior ;-)