While addressing ASDF [issue
#140](https://gitlab.common-lisp.net/asdf/asdf/-/issues/140) I stumbled
on what seems to me to be an error. That is that
(ensure-directory-pathname "")
at least on SBCL, returns a `pathname` that has `NIL` in all its slots
except `HOST`.
To me, this seemed wrong, and looking further, I found that
`directory-pathname-p` returns `T` when given this pathname, because
`directory-pathname-p` is defined (in its docstring) as:
> A directory-pathname is a pathname _without_ a filename.
I would have assumed that a directory pathname was *also* a pathname
with a non-`NIL` `directory` component.
I am considering making this (incompatible) change, but am interested in
any feedback about whether this is the correct thing to do.
