Resolve #3546262 "Just the fix"

Closes #3546262

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.