Cover: Improve disabled media buttons check for placeholder

[Mamaduka]

Description

Prevents placeholder from being rendered, when using cover block without media. See #29813.

Thanks, @jasmussen for catching this issue.

How has this been tested?

1. Add cover block.
2. Select color.
3. See if the placeholder is rendered.

Types of changes

Bugfix.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[jasmussen]

Beautifully small changeset, and works for me. Thank you for fast work 👌

Before:

After:

[getsource]

Tested, and works here with the same results as well. Thanks!

[ajitbohra]

Works as expected 👍