Strip null values from styles when saving to theme.json #885

Description

When the user's global styles contain null values, "Save Changes to Theme" writes them into theme.json as they are. The theme.json schema rejects null here, and WordPress ignores these keys, so they're only noise that theme authors have to remove by hand.

The editor currently saves such nulls when border sides are unlinked in Global Styles (WordPress/gutenberg#84814), for example:

"core/quote": {
  "border": {
    "color": null,
    "style": null,
    "width": null,
    "left": { "color": "currentColor", "style": "solid", "width": "4px" }
  }
}

Steps to reproduce

1. Activate Twenty Twenty-Five with Create Block Theme.
2. Site Editor → Styles → Blocks → Quote → Borders: click "Unlink sides", set the left width to 4px, save.
3. Save Changes to Theme (or call CBT_Theme_JSON_Resolver::export_theme_data( 'all' )).

Expected

null values are dropped from the exported theme.json (and style variations).

Actual

styles.blocks["core/quote"].border in the export has "color":null, "style":null and "width":null.

Environment

• Create Block Theme 2.10.2
• WordPress 7.1.3, Gutenberg 24.2.0

Drafted with AI assistance, reproduced and verified by me.