Flush headers after null StreamingResponseBody

Issue: SPR-14315

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

@@ -74,6 +74,8 @@ public class StreamingResponseBodyReturnValueHandler implements HandlerMethodRet
 			returnValue = responseEntity.getBody();
 			if (returnValue == null) {
 				mavContainer.setRequestHandled(true);
+				// Ensure headers are flushed
+				outputMessage.flush();
 				return;
 			}
 		}