Complete empty Uni instances from the Mutiny reactive adapter

[junhyeong9812]

Overview

The Mutiny Uni adapter in ReactiveAdapterRegistry supplies Uni.createFrom().nothing() as its empty value. That Uni never signals, so everywhere the framework adapts a null source - most visibly a WebFlux handler method declared to return Uni<T> that returns null - the resulting Publisher never completes and the request hangs, while the same handler declared with Mono<T> completes empty. This PR changes the supplier to Uni.createFrom().nullItem().

Problem

ReactiveAdapter.toPublisher(Object) substitutes getDescriptor().getEmptyValue() when the source is null. The Uni descriptor is registered with ReactiveTypeDescriptor.singleOptionalValue, which declares that the type supports an empty value, but the supplied empty value is Uni.createFrom().nothing() - a Uni that never emits an item, a failure, or completion. Adapting it produces a Publisher that never signals, so a blocking or subscribing consumer waits until its own timeout.

All sibling registrations supply an empty value that completes immediately: Mono::empty, Maybe::empty, Completable::complete, CompletableDeferred(null), and the Multi registration itself uses Multi.createFrom().empty(). The Uni registration is also asymmetric with its own fromPublisher function: adapting an empty Publisher produces a Uni that completes with a null item, but the registered empty value cannot make the trip back.

Fix

The empty-value supplier now uses Uni.createFrom().nullItem(). Mutiny converts a Uni with a null item to a Publisher that completes without emitting an item (Reactive Streams forbids onNext(null)), which matches the sibling adapters and makes the round trip through fromPublisher symmetric. The descriptor instance is shared by the Mutiny 1 and Mutiny 2 registrations, so both are covered.

Two tests are added: adapting a null source completes empty within a timeout (previously it hung), and adapting an empty Publisher to Uni yields a null item, pinning the round-trip contract. The full spring-core test suite passes.

[bclozel]

Thanks @junhyeong9812 !