LibXML: Notify the listener about the root node as well

We previously did not notify the listener about entering the root node,
which caused the following snippet to produce the wrong output:
    a = new DOMParser
    a.parseFromString("<x/>", "text/xml").documentElement // != null

Userland/Libraries/LibXML/Parser/Parser.cpp

@@ -74,7 +74,7 @@ void Parser::append_node(NonnullOwnPtr<Node> node)
         m_entered_node->content.get<Node::Element>().children.append(move(node));
     } else {
         m_root_node = move(node);
-        m_entered_node = m_root_node.ptr();
+        enter_node(*m_root_node);
     }
 }