Fix the 'isUnsubscribed' issue for Subscrptions.empty()

As per #1943
Why does this mean `true` now whereas it didn't before?

If anyone is using `Subscriptions.empty()` as a no-op it will now kill their code and stop processing so this could be a breaking change so I don't see this as a trivial change. 

I don't see how "empty" means "unsubscribed", it means it "does nothing". Returning "true" on `isUnsubscribed" now does "something".