Cover block hides content if color variable is not set

[supernovia]

Description

A user created a cover block with fairly complex content. When they switched themes, there were no obvious errors in the editor, but none of the content appeared either. I took video so you can get a better idea what's happening:
https://cloudup.com/cHLs8znxI1n

HT @akirk for figuring out what was causing the user's content to go missing!

Step-by-step reproduction instructions

• Activate a theme that has theme-based color support (this user had Hever active)
• Start a post or page. Add a cover block and pick a background color from the theme's palette
• Add content to the cover block and publish
• Switch themes to one that does not have the same type of color support
• Edit the post; note the visual editor does not display the content, even though it is still there in "Code" as well as on preview.

Expected behaviour

I would expect the cover block to display, perhaps with default colors selected.

Actual behaviour

The cover block shows a color selection option and does not display the content.

Screenshots or screen recording (optional)

https://cloudup.com/cHLs8znxI1n

WordPress information

• WordPress version: Sorry I'm not finding this, but it's on a WordPress.com free site, so everything is fairly current.

Device information

• Device: n/a, it happens on various devices, OSs, and browers.
• Operating system: n/a
• Browser: n/a
• User report here: https://wordpress.com/forums/topic/visual-editor-doesnt-function/#post-3669988

[supernovia]

Noting this possibly related discussion: #29568

[stokesman]

This is another way to reproduce #28888. Switching themes and having the content not show seems like it would be especially confusing.