addWatch api should check the null watch

public void addWatch(String basePath, Watcher watcher, AddWatchMode mode)
        throws KeeperException, InterruptedException {
    PathUtils.validatePath(basePath);
    String serverPath = prependChroot(basePath);

    RequestHeader h = new RequestHeader();
    h.setType(ZooDefs.OpCode.addWatch);
    AddWatchRequest request = new AddWatchRequest(serverPath, mode.getMode());
    ReplyHeader r = cnxn.submitRequest(h, request, new ErrorResponse(),
    

zk.addWatch("/a/b", null, PERSISTENT_RECURSIVE);