We need to remove empty values that may occur when the user resets a value.
These empty values prevent applying defaults coming from editor theme after reset.
[MAILPOET-6335]
Some components in the style setting (e.g., SpacingSizesControl) don't work
with the CSS variables when present in valid CSS form (e.g., var(--wp--preset--spacing--10)).
To prevent issues, we need to transform CSS variables to the shortened variant.
The core uses more sophisticated functions for that, see
f2ba0fdb4c/packages/block-editor/src/components/global-styles/utils.js (L307-L358)
The functionality is private, so I've created a simpler version.
[MAILPOET-6335]
