Make "empty path element" handling more explicit.

```python
>>> a = URI("http://example.com/foo//bar/"); a
URI('http://example.com/foo/bar/')
```

```python
>>> a / "" / "baz"
URI('http://example.com/foo/bar/baz')
```

```python
>>> a / "//baz"
URI('http://example.com//baz')
```

Empty path segments must be allowed, and should be easier to work with / induce than including path element separators within a path segment.