...when path contains no hash-path-separator ('/')
This is a workaround to #17179.
Entering this case when `path` does not contain a '/' does not really
make sense, as that means the tree path is empty, but this case is only
entered for routes that expect a non-empty tree path.
Treepaths like <40-char-dirname>/<filename> will still fail,
but hopefully don't occur that often. A more complete fix that avoids
this case too is outlined in #17185, but too big of a change to backport
245596e130