Define behavior of null Callable or DeferredResult

When a controller method declares Callable or DeferredResult as its
return value, and returns null, async processing will not start and
the request will be considered "handled" by the controller method.

Issue: SPR-9951

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/AsyncTaskMethodReturnValueHandler.java

@@ -49,6 +49,7 @@ public class AsyncTaskMethodReturnValueHandler implements HandlerMethodReturnVal
 			NativeWebRequest webRequest) throws Exception {
 
 		if (returnValue == null) {
+			mavContainer.setRequestHandled(true);
 			return;
 		}
 

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/CallableMethodReturnValueHandler.java

@@ -42,6 +42,7 @@ public class CallableMethodReturnValueHandler implements HandlerMethodReturnValu
 			NativeWebRequest webRequest) throws Exception {
 
 		if (returnValue == null) {
+			mavContainer.setRequestHandled(true);
 			return;
 		}
 

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/DeferredResultMethodReturnValueHandler.java

@@ -41,6 +41,7 @@ public class DeferredResultMethodReturnValueHandler implements HandlerMethodRetu
 			NativeWebRequest webRequest) throws Exception {
 
 		if (returnValue == null) {
+			mavContainer.setRequestHandled(true);
 			return;
 		}