BodyObservable: when null value slips into RxJava2 NPE is not propagated to onError()

[southerton81]

When the response came with empty body, Retrofit BodyObservable calls onNext() with null value. In RxJava2 that leads to NPE, but that is not an issue, and there's a recommended way to deal with it using Completable or Optional converter (https://github.com/square/retrofit/issues/2242).

The problem I have encountered is that onError() and doFinally() handlers are not triggered at all, so that observable silently fails when getting NPE (making it harder to spot and fix). I've managed to isolate this case:

https://gist.github.com/southerton81/7cdbfd10c59c7b58b7ad468c27dcb9b0