[*]返回原来的name，而不是null，避免NullPointerException

src/main/java/com/alibaba/druid/pool/DruidDataSource.java

@@ -2728,7 +2728,9 @@ public class DruidDataSource extends DruidAbstractDataSource
 
     @Override
     public ObjectName preRegister(MBeanServer server, ObjectName name) throws Exception {
-        return null;
+        //do nothing
+        //return original name to avoid NullPointerException
+        return name;
     }
 
     @Override