HasAttribute should only check nullity

gwtquery-core/src/main/java/com/google/gwt/query/client/js/JsUtils.java

@@ -342,7 +342,7 @@ public class JsUtils {
    * present.
    */
   public static native boolean hasAttribute(Element o, String name) /*-{
-    return !!(o && o.getAttribute(name));
+    return !!(o && o.getAttribute(name) !== null);
   }-*/;
 
   /**