Empty CSS custom property values are silently dropped

[quantizor]

Description

Stylis silently drops CSS declarations where a custom property has an empty value (--prop: ;). Per the CSS spec, --custom-property: ; is valid — it sets the property to the guaranteed-invalid value. This pattern is used in scroll-driven animations and other modern CSS techniques.

Reproduction

import { compile, serialize, stringify } from 'stylis';

// Empty value — dropped entirely
console.log(serialize(compile('.foo { --x: ; }'), stringify));
// Output: "" (empty string)

// In keyframes — entire frame dropped
console.log(serialize(compile('@keyframes foo { from { --x: red; } to { --x: ; } }'), stringify));
// Output: "@keyframes foo{from{--x:red;}}" (the `to` frame is missing)

// Non-empty value — works fine
console.log(serialize(compile('.foo { --x: initial; }'), stringify));
// Output: ".foo{--x:initial;}"

Expected

.foo{--x: ;}
@keyframes foo{from{--x:red;}to{--x: ;}}

Actual

(empty)
@keyframes foo{from{--x:red;}}

Environment

stylis 4.3.6

[Andarist]

duplicate of #328 , I'm not sure why @thysultan decided to only preserve the ones with comments