Spring MVC supports reactive return values

This commit adds support for reactive library types to be returned
directly from controller methods adapting them either to a
ResponseBodyEmitter (streaming) or DeferredResult (non-streaming).

The reactive libraries supported are the ones that can adapted to a
Reactive Streams Publisher through the ReactiveAdapterRegistry.

Issue: SPR-15365

spring-core/src/main/java/org/springframework/core/ReactiveAdapterRegistry.java

@@ -78,6 +78,16 @@ public class ReactiveAdapterRegistry {
 	}
 
 
+	/**
+	 * Whether the registry has any adapters which would be the case if any of
+	 * Reactor, RxJava 2, or RxJava 1 (+ RxJava Reactive Streams bridge) are
+	 * present on the classpath.
+	 */
+	public boolean hasAdapters() {
+		return !this.adapters.isEmpty();
+	}
+
+
 	/**
 	 * Register a reactive type along with functions to adapt to and from a
 	 * Reactive Streams {@link Publisher}. The functions can assume their
@@ -113,6 +123,10 @@ public class ReactiveAdapterRegistry {
 		Object sourceToUse = (source instanceof Optional ? ((Optional<?>) source).orElse(null) : source);
 		Class<?> clazz = (sourceToUse != null ? sourceToUse.getClass() : reactiveType);
 
+		if (reactiveType == null) {
+			return null;
+		}
+
 		return this.adapters.stream()
 				.filter(adapter -> adapter.getReactiveType() == clazz)
 				.findFirst()