Differentiate from zero-sized fragment and no fragment in url

```
scheme://host:port/#
```
and
```
scheme://host:port/
```
if fed to the `URL` do not distinguish between the two: URL.hash returns `''`

and to make it even stranger passing `.hash = '#'` produces `scheme://host:port/#` but calling `.hash` returns `''` nonetheless.

would be nicer if `.hash` returns `undefined`/`null` if it is unset or `"#"` if the trailing hash is present.
