stab doesn't parse empty path properly

```
> `(pole span)`(stab '/')
[~. ~]
```

expected: `~`

EDIT (2019/06/24):

Still an issue.  Using `knot` instead of `span`:

```
> `(pole knot)`(stab '/')
[~. ~]
> `(pole knot)`/
~
> `(pole knot)`~
~
> `(list knot)`(stab '/')
~
```

