protected Iterator<N> getCurrentChildrenIterator() {
    if (visitorStack.isEmpty()) {
      return null;
    } else {
      return visitorStack.peek().iterator.getIterator();
    }
  }
Get current children iterator generated by {@link AbstractTreeVisitor#getChildrenIterator}

@return null if there is no current children iterator
iotdb-core/node-commons/src/main/java/org/apache/iotdb/commons/schema/tree/AbstractTreeVisitor.java