visibility cycling: if subtree has no children, cycle to folded state

[notestaff]

Visibility-cycling a headline that is open and has no children should go directly to folded state, rather than doing nothing.

[tj64]

Reasonable request, but did not make it at the top of my todo-list yet, since developing outshine towards a kind-of org-minor-mode has priority right now. I keep the issue open and keep an eye on it.