Disable duotone until media is provided in Cover block

[jrtashjian]

What?

Within the Duotone control hook, assess specific blocks (cover and image) and associated media attributes to determine whether Duotone block controls should be displayed. It's worth noting that, since this feature is enabled through block.json, there's no known method for other blocks to conditionally render these controls in a similar manner.

Why?

Resolves #48804

How?

By looking at the block name and the media attributes we know.

Even though this works for now, it might not work well in the future. Media attributes can vary a lot and aren't standardized, making it tricky to set conditions.

Screenshots or screencast

New cover block

Cover block with useFeaturedImage

Cover block with media id

Cover block with overlay color only

New image block

Image block with media id

[ajlende]

The image block has a visual placeholder when the block isn't selected. It's useful to set the duotone filter before adding an image for patterns. The same thing exists for previewing borders on the placeholder.

Instead of disabling duotone, could we provide a better placeholder for the cover block like we have for the image block?

There's still the problem of having a color/gradient background with a duotone filter that does nothing. But I think a satisfactory solution for that would be simply clearing the duotone filter when a color/gradient background is selected.