code: Map<String, UserData> getUserDataMapForRead(NodeImpl node) {
          if (nodeToUserData == null) {
              return Collections.emptyMap();
          }
          Map<String, UserData> userDataMap = nodeToUserData.get(node);
          return userDataMap == null
                  ? Collections.<String, UserData>emptyMap()
                  : userDataMap;
      }
docs: Returns a map with the user data objects attached to the specified node.
  The returned map may be read-only.