code: @SuppressWarnings("unchecked")
      static <T> Node<T> emptyNode(StreamShape shape) {
          switch (shape) {
              case REFERENCE:    return (Node<T>) EMPTY_NODE;
              case INT_VALUE:    return (Node<T>) EMPTY_INT_NODE;
              case LONG_VALUE:   return (Node<T>) EMPTY_LONG_NODE;
              case DOUBLE_VALUE: return (Node<T>) EMPTY_DOUBLE_NODE;
              default:
                  throw new IllegalStateException("Unknown shape " + shape);
          }
      }
docs: Produces an empty node whose count is zero, has no children and no content.
  
  @param <T> the type of elements of the created node
  @param shape the shape of the node to be created
  @return an empty node.