Don't allow interpolating SVGPaintKind::None.

PR #18103 disallowed interpolation between fill:none and fill:none, but that change was regressed by the refactoring in PR #18239.
This patch restores the intended behavior, disabling animation of SVGPaintKind::None.

components/style/values/generics/svg.rs

@@ -35,6 +35,7 @@ pub struct SVGPaint<ColorType, UrlPaintServer> {
 #[derive(ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
 pub enum SVGPaintKind<ColorType, UrlPaintServer> {
     /// `none`
+    #[animation(error)]
     None,
     /// `<color>`
     Color(ColorType),