Replaced a java 6 method with a java 5 equivalent

git-svn-id: https://svn.apache.org/repos/asf/hadoop/zookeeper/trunk@678868 13f79535-47bb-0310-9956-ffa450edef68

src/java/jmx/org/apache/zookeeper/jmx/MBeanRegistry.java

@@ -143,7 +143,7 @@ public class MBeanRegistry {
     private int tokenize(StringBuilder sb, String path, int index){
         String[] tokens = path.split("/");
         for (String s: tokens) {
-            if (s.isEmpty())
+            if (s.length()==0)
                 continue;
             sb.append("name").append(index++)
                     .append("=").append(s).append(",");