Fix issue with defaults not being applied for dynamic blocks

[talldan]

Description

#16796 introduced a regression where default attributes for dynamic blocks stopped working. This change resolves that issue.

Also fixes #15487.

How has this been tested?

I looked into adding an e2e test. It's a definite possibility to test a block like the Search block, but we'd need to produce a snapshot comparing against the rendered version of a post rather than the raw version like we do for static blocks.

For now, manual testing steps:

1. Create a new post, add a search block
2. Preview the post
3. Expect that the search block should render correctly in the post

Screenshots

Before

After

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[noisysocks]

Good catch, thanks for fixing!

[youknowriad]

@talldan do you know if this change have been packported into Core (the php change) or whether it's included in the last patch here https://core.trac.wordpress.org/ticket/47843

cc @gziolo

[talldan]

@youknowriad Sorry, I don't know.