List view: Show placeholder material when a block is in placeholder state

[jasmussen]

When an image block is in a placeholder state, it shows a literal gray-with-diagonal-line placeholder material to indicate what it is. Shown here, an image, and a placeholder image:

In the list view, images have little thumbnails that show up next to them:

Because images in a placeholder state are not that useful, it might be useful to further highlight that, by showing the same placeholder material for blocks in said state:

@WordPress/gutenberg-design

[hanneslsm]

One argument against adding a placeholder thumbnail in the list view is that blocks without content are not rendered on the front end.
I feel like adding a thumbnail would not differentiate the block enough from ones that have content.
However, additionally the Image Block (and any other blocks without content) could be greyed out in the list view to indicate that they need to be filled.

[richtabor]

One argument against adding a placeholder thumbnail in the list view is that blocks without content are not rendered on the front end.

I wonder if there should be a condition where image blocks without images do render on the front-end. Like if there's an aspect ratio, or a hidden attribute. That way you could share designs with folks, even without imagery.

However, additionally the Image Block (and any other blocks without content) could be greyed out in the list view to indicate that they need to be filled.

But then it may just as well feel disabled, indicating you can't upload an image.

[jasmussen]

I wonder if there should be a condition where image blocks without images do render on the front-end. Like if there's an aspect ratio, or a hidden attribute. That way you could share designs with folks, even without imagery.

Would that be a prerequisite for this issue, you think? And it sounds useful to me. Can even be something like <img data-is-placeholder alt="" [...] />, with a special CSS style for the empty image.