Fixing issue with rxjava2/3 based Promise implementations not handling null

[davydotcom]

It can be often the case that a Promise is used for the purposes of asynchronous operations in an isolated thread / persistence interceptor context. These async operations may not even care about a return type because they may be kicking off other error handling behaviors. In the case of the rxjava based implementations, rxjava gets angry when a return is null based and throws several exceptions:

Example:

RxPromiseSpec > Test promise null handling FAILED
    java.lang.NullPointerException: onSuccess called with null. Null values are generally not allowed in 2.x operators and sources.
        at io.reactivex.internal.operators.single.SingleCreate$Emitter.onSuccess(SingleCreate.java:65)
        at org.grails.async.factory.rxjava2.RxPromise$_closure1.doCall(RxPromise.groovy:47)
        at groovy.lang.Closure.call(Closure.java:433)
        at io.reactivex.internal.operators.single.SingleCreate.subscribeActual(SingleCreate.java:39)
        at io.reactivex.Single.subscribe(Single.java:3666)
        at io.reactivex.internal.operators.single.SingleSubscribeOn$SubscribeOnObserver.run(SingleSubscribeOn.java:89)
        at io.reactivex.Scheduler$DisposeTask.run(Scheduler.java:608)
        at io.reactivex.internal.schedulers.ScheduledRunnable.run(ScheduledRunnable.java:66)
        at io.reactivex.internal.schedulers.ScheduledRunnable.call(ScheduledRunnable.java:57)
        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
        at java.base/java.lang.Thread.run(Thread.java:840)

This PR resolves this by translating null to a Void object type which allows this to pass through and behave like other Promise implementations.

[matrei]

Nice! Added some comments on the tests.

[jdaugherty]

Should we remove rxjava1 since we have 2 + 3 now?

[davydotcom]

Should we remove rxjava1 since we have 2 + 3 now?

rxjava1 is well beyond end of life, I believe. I'm open for deprecation and maybe future removal or just removal.