Blend.Single changes

[OttoHatt]

trafficstars

• Allow returning nil from the observable. We know that it will never evaluate to anything interesting, so there's no point creating a brio (which are not free). (Should also probably skip creating a brio if the passed observable is complete and evaluates to nil, i.e. == Rx.EMPTY?).
• Propagate state of passed observable. This has just become a slightly more efficient version of a switchMap + toBrio.
• Returning from a subscription does nothing.

[Quenty]

Side note: Rx.EMPTY never completes I believe.

[OttoHatt]

If propagating failure is contentious, is it possible to merge just the first of these commits for now?