Serialize attribute nodes as the empty string (#36875)

The existing code asserts that attribute nodes are never serialized.
This is wrong, because you can pass an attribute node to
`XMLSerializer::serializeToString`. Instead, the spec mandates that
these are serialized as empty strings
(https://w3c.github.io/DOM-Parsing/#dfn-xml-serialization-algorithm).

Testing: Includes a new web platform test
Fixes: https://github.com/servo/servo/issues/36872

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/script/dom/servoparser/html.rs

@@ -302,11 +302,10 @@ pub(crate) fn serialize_html_fragment<S: Serializer>(
                     serializer.write_processing_instruction(pi.target(), &data)?;
                 },
 
-                NodeTypeId::DocumentFragment(_) => {},
+                NodeTypeId::DocumentFragment(_) | NodeTypeId::Attr => {},
 
                 NodeTypeId::Document(_) => panic!("Can't serialize Document node itself"),
                 NodeTypeId::Element(_) => panic!("Element shouldn't appear here"),
-                NodeTypeId::Attr => panic!("Attr shouldn't appear here"),
             },
             SerializationCommand::SerializeShadowRoot(shadow_root) => {
                 // Shadow roots are serialized as template elements with a fixed set of