Cover Block: Prevent styles from applying to the placeholder

[WunderBart]

Describe the bug
The Styles setting for the Cover block should not be available initially as they can mask the placeholder interface and cause confusion (especially when one of the "wave" options is selected as a block's default)

To reproduce
Steps to reproduce the behavior:

1. Add Cover block,
2. Open the block settings,
3. Go to the Styles section,
4. Apply one of the "wave" styles.
5. See how the placeholder gets masked with that wave shape.

Expected behavior
Styles should not be applicable to the block placeholder. Ideally, they should not appear until a block is set up.

Screenshots

Editor version:
v8.3.0

[richtabor]

Related to #23122

[richtabor]

Also, the Bottom Wave and Top Wave styles here are added via a third party block (presumably CoBlocks) - not to say the issue is any less important (as developers continue to extend the editor).

In #23122 I propose that the Styles panels are not available for blocks that utilize placeholder states, so that this issue does not occur.

[jordesign]

This is still somewhat present. The Styles panel is available but doesn't impact the block until you add a background color - at which point the placeholder state is lost

[akasunil]

based on comment of @getsource in #23122, should we close this issue ?