Issue #3546262 by mherchel, jessebaker: Conditional slots: changing props that then show/hide slots does not update the UI as expected

Closes #3546262

THIS MR SHOULD NOT BE MERGED - IT CONTAINS A TEST SDC JUST FOR TRYING OUT THIS ISSUE. Instead, if approved, merge !210 (closed)

Relatively trivial fix - before it was always assumed there would be a 1:1 of slots in the layout JSON:slot elements in the preview HTML.

In this case, if I understand the issue correctly, the slots in the model JSON are always there, but the Twig of the SDC is rendering only some of them (based on a number in a prop). The overlay which renders the borders etc wasn't smart enough to render nothing in that situation.

The second commit in this MR just adds a very simple SDC that exhibits the issue.