Delete EmptyObserver

It creates no value and confuses with Observers.empty() which throws an OnErrorNotImplementedException if onError is invoked.
Fixes https://github.com/ReactiveX/RxJava/issues/1741
Clarify Javadoc on Observers.empty()

It throws an Exception ... and now EmptyObserver is gone.
Swallow Errors in TestObserver

If a delegate is not provided then do nothing.
Fixes https://github.com/ReactiveX/RxJava/issues/1571
Merge pull request #1742 from benjchristensen/issue-1571

EmptyObserver and TestObserver