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;
 			}
 		}

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitterReturnValueHandlerTests.java

@@ -21,6 +21,7 @@ import static org.springframework.web.servlet.mvc.method.annotation.SseEmitter.*
 
 import java.lang.reflect.Method;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicReference;
 
@@ -184,11 +185,12 @@ public class ResponseBodyEmitterReturnValueHandlerTests {
 	@Test
 	public void responseEntitySseNoContent() throws Exception {
 		MethodParameter returnType = returnType("handleResponseEntitySse");
-		ResponseEntity<?> entity = ResponseEntity.noContent().build();
+		ResponseEntity<?> entity = ResponseEntity.noContent().header("foo", "bar").build();
 		handleReturnValue(entity, returnType);
 
 		assertFalse(this.request.isAsyncStarted());
 		assertEquals(204, this.response.getStatus());
+		assertEquals(Collections.singletonList("bar"), this.response.getHeaders("foo"));
 	}
 
 	private void handleReturnValue(Object returnValue, MethodParameter returnType) throws Exception {