Global Styles: Fix blocks custom css selectors and empty rules

[aaronrobertshaw]

Syncs the changes from WordPress/gutenberg#63980

These updates include the PHP fixes for empty CSS rules and invalid selectors for Global Styles custom CSS for blocks.

Please note: JS package updates will be required to resolve the issue in the editor.

Testing Instructions

1. In the site editor navigate to Styles > Blocks > Heading > Advanced
2. Add the following into the Custom CSS field: &::before { content: 'Foo' }
3. Ensure the page being edited has a heading block
4. Save and view the frontend
5. Confirm the heading block has a pseudo element displaying the word "Foo".

Screenshots or screencast

Before	After

Before	After

Trac ticket: https://core.trac.wordpress.org/ticket/61769

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props aaronrobertshaw, andrewserong, aristath, ramonopoly.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[andrewserong]

This is testing well for me:

✅ The logic looks good to me (only applies regex to the already split out nested selector, and appends any matched pseudo elements to the end)
✅ Test coverage looks good
✅ Works nicely on the site frontend
✅ Existing rules that don't use a selector (root CSS) continue to work as expected (e.g. placing background-color: red under the Heading block's Advanced area works as on trunk)

As discussed over in WordPress/gutenberg#63980 (comment), this fix focuses on fixing a particular issue rather than handling all possible cases in CSS 👍

LGTM! 🚀

[ramonjd]

I tested along the lines of WordPress/gutenberg#63980 also LGTM

Thanks for reviewing so quickly, folks!

[hellofromtonya]

• ✅ Tests well for me (see test report)
• ✅ All PHP changes from the original Gutenberg PR are included.

LGTM for commit 👍

[hellofromtonya]

Committed to trunk via https://core.trac.wordpress.org/changeset/58896.