Layout: Fix invalid css for nested fullwidth layouts with zero padding applied

[andrewserong]

Backport WordPress/gutenberg#63436 to core.

In the Layout block support, handle 0 values for padding as 0px in calc() rules. This resolves a bug for nested fullwidth layouts when zero padding is applied. Due to how calc() works, without supplying the unit, the rule will not work, resulting in a horizontal scrollbar. For more information see the original issue in the Gutenberg repo, or the fix.

Testing instructions

1. Open a post or page.
2. Open the code editor within the top editor options panel.
3. Copy and paste this gist, which configures the conditions properly (nested align full blocks, with padding zeroed out on the parent).
4. See no horizontal scroll in the site front end.

Note: the issue will still be present in the post editor until the JS packages that include the JS changes from WordPress/gutenberg#63436 are included in core.

Trac ticket: https://core.trac.wordpress.org/ticket/61656

Props: @richtabor for the fix.

---

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 andrewserong, mukesh27.

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.

[mukeshpanchal27]

Thanks @andrewserong, The changes look good to me.

[aaronrobertshaw]

Thanks for this @andrewserong 👍

✅ PHP changes match WordPress/gutenberg#63436
✅ No horizontal scrollbar on the frontend

LGTM!

[noisysocks]

Committed in r58750 and re-opened https://core.trac.wordpress.org/ticket/61656 to request backport to the 6.6 branch.