Fix upward path search operator without lefthand side

[ike709]

The upward path search operator didn't resolve paths correctly specifically when there was nothing on the lefthand side of the operator.

This is because:

/obj/foo
/obj/bar
    var/meep = .foo

would resolve to /obj/.foo instead of /obj/foo

Now we strip the search operator out of the path string if there's nothing on the lefthand side, so it can be combined into the correct path.

The other upward search unit tests confirm that this doesn't break anything when there is a lefthand side. I also confirmed TG didn't explode.

Fixes #617