Allow empty media query list

And make empty list the default value of MediaList.

This commit also removes Option wrapper of Stylesheet.media because
a stylesheet should always have an associated media query list.

components/script/dom/htmlmetaelement.rs

@@ -83,7 +83,7 @@ impl HTMLMetaElement {
                     *self.stylesheet.borrow_mut() = Some(Arc::new(Stylesheet {
                         rules: vec![CSSRule::Viewport(Arc::new(RwLock::new(translated_rule)))],
                         origin: Origin::Author,
-                        media: None,
+                        media: Default::default(),
                         // Viewport constraints are always recomputed on resize; they don't need to
                         // force all styles to be recomputed.
                         dirty_on_viewport_size_change: false,