Document `Session.is_empty`

[Jake Howard]

It's often useful to check whether the user has a session, for example to avoid creating one unnecessarily. Since request.session is always populated when SessionMiddleware is used, it's better to check whether the session is empty. BaseSession has a is_empty method, but it's undocumented.

It should be documented.

Additionally, perhaps BaseSession.__bool__ should exist, so if request.session works as expected? The downside being if getattr(request, "session", None) would be False when sessions are being used, but the session is empty (arguably hasattr would be better there anyway).