LibWeb: Make the default XML namespace an empty Optional

Previously, when constructing an XML document, the default namespace
was the empty string. This led to XML documents having empty xmlns
attributes when serialized.

Userland/Libraries/LibWeb/XML/XMLDocumentBuilder.h

@@ -42,10 +42,10 @@ private:
     XMLScriptingSupport m_scripting_support { XMLScriptingSupport::Enabled };
     bool m_has_error { false };
     StringBuilder text_builder;
-    FlyString m_namespace;
+    Optional<FlyString> m_namespace;
 
     struct NamespaceStackEntry {
-        FlyString ns;
+        Optional<FlyString> ns;
         size_t depth;
     };
     Vector<NamespaceStackEntry, 2> m_namespace_stack;