Previews of SDCs with default image props broken

Turns out the fix was trivial, and thanks to the combination of #3518832 + #3518253, this was trivial to add test coverage for!

Landing because the change is trivial, is well-tested, and is blocking 📌 [later phase] Support matching `{type: array, …}` prop shapes Postponed (where I discovered this). See #3467870-27: Support `{type: array, …}` prop shapes → — a preview of an image gallery SDC with default images! 🥳

(I'd have loved @longwave's +1, but it's a bank holiday in the UK and he's the only one who knows the touched LoC well.)