Treat empty paths correctly

[udoprog]

The way that to_path and to_logical_path reacts to empty paths wasn't properly specified nor implemented. The current behavior ends up being rather on, in that the main separator is added so that the following would produce an absolute path on Linux:

let path = RelativePath::new("foo/bar").to_path("");
assert_eq!(Path::new("/foo/bar"), path);

This is clearly dysfunctional and contrary to what this crate is trying to accomplish. So this patch changes this behavior so that we only add the main separator for cases where the from path is non-empty.

What we then get instead is:

let path = RelativePath::new("foo/bar").to_path("");
assert_eq!(Path::new("foo/bar"), path);