Flush headers after null ResponseBodyEmitter

Issue: SPR-14315

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

@@ -137,6 +137,7 @@ public class ResponseBodyEmitterReturnValueHandler implements AsyncHandlerMethod
 			returnValue = responseEntity.getBody();
 			if (returnValue == null) {
 				mavContainer.setRequestHandled(true);
+				outputMessage.flush();
 				return;
 			}
 		}