Clean up empty namespace handling and fix empty namespaces in compound selectors

Source/core/css/StyleSheetContents.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 256 matching lines @@
     }
     PrefixNamespaceURIMap::AddResult result = m_namespaces.add(prefix, uri);
     if (result.isNewEntry)
         return;
     result.storedValue->value = uri;
 }
 
 const AtomicString& StyleSheetContents::determineNamespace(const AtomicString& prefix)
 {
     if (prefix.isNull())
-        return nullAtom; // No namespace. If an element/attribute has a namespace, we won't match it.
+        return defaultNamespace();
+    if (prefix.isEmpty())
+        return emptyAtom; // No namespace. If an element/attribute has a namespace, we won't match it.
     if (prefix == starAtom)
         return starAtom; // We'll match any namespace.
     return m_namespaces.get(prefix);
 }
 
 void StyleSheetContents::parseAuthorStyleSheet(const CSSStyleSheetResource* cachedStyleSheet, const SecurityOrigin* securityOrigin)
 {
     TRACE_EVENT1("blink,devtools.timeline", "ParseAuthorStyleSheet", "data", InspectorParseAuthorStyleSheetEvent::data(cachedStyleSheet));
 
     bool isSameOriginRequest = securityOrigin && securityOrigin->canRequest(baseURL());
@@ skipping 342 matching lines @@
     visitor->trace(m_ownerRule);
     visitor->trace(m_importRules);
     visitor->trace(m_childRules);
     visitor->trace(m_loadingClients);
     visitor->trace(m_completedClients);
     visitor->trace(m_ruleSet);
 #endif
 }
 
 }