nodeplotblank: Function to avoid plotting node labels
Description
Unlike, nodeplotdefault and nodeplotboot,
this function does not return a function, but instead is provided
directly to the nodelabf argument of plot_tree to
ensure that node labels are not added to the graphic.
Please note that you do not need to create or obtain the arguments to
this function. Instead, you can provide this function directly to
plot_tree and it will know what to do with it. Namely,
use it to avoid plotting any node labels.