Treat empty URL as the "default" namespace.

src/element.rs

@@ -388,6 +388,9 @@ impl Element {
 
     /// Get namespace value given prefix, for this element.
     /// "xml" and "xmlns" returns its default namespace.
+    ///
+    /// This method can return an empty namespace, but only for an empty prefix assuming
+    /// there is no default namespace declared.
     pub fn namespace_for_prefix<'a>(&self, doc: &'a Document, prefix: &str) -> Option<&'a str> {
         match prefix {
             "xml" => return Some("http://www.w3.org/XML/1998/namespace"),
@@ -400,7 +403,13 @@ impl Element {
             if let Some(value) = data.namespace_decls.get(prefix) {
                 return Some(value);
             }
-            elem = elem.parent(doc)?;
+            if let Some(parent) = elem.parent(doc) {
+                elem = parent;
+            } else if prefix.is_empty() {
+                return Some("");
+            } else {
+                return None;
+            }
         }
     }