Defensively register ReactiveReturnValueHandler for messaging methods

[jhoeller]

As of our support for reactive return types from messaging handler methods in #21175, we register a ReactiveReturnValueHandler which has a reference to the ReactiveAdapterRegistry type, which in turn refers to ReactiveAdapter which has a hard reference to org.reactivestreams.Publisher. In order to avoid class resolution failures in certain JVM scenarios (in particular in debug mode), we should defensively register the ReactiveReturnValueHandler only when the Reactive Streams API is present on the classpath.