INT-4180: RecipientListRouterSpec Fix

JIRA: https://jira.spring.io/browse/INT-4180

Use `if...else` for `null` in `recipient()` with `Expression`.

Add test.

spring-integration-core/src/main/java/org/springframework/integration/dsl/RecipientListRouterSpec.java

@@ -32,6 +32,7 @@
  * An {@link AbstractRouterSpec} for a {@link RecipientListRouter}.
  *
  * @author Artem Bilan
+ * @author Gary Russell
  *
  * @since 5.0
  */
@@ -142,9 +143,14 @@ public RecipientListRouterSpec recipient(MessageChannel channel, String expressi
 	 * @return the router spec.
 	 */
 	public RecipientListRouterSpec recipient(MessageChannel channel, Expression expression) {
-		ExpressionEvaluatingSelector selector = new ExpressionEvaluatingSelector(expression);
-		this.target.addRecipient(channel, selector);
-		this.componentsToRegister.add(selector);
+		if (expression != null) {
+			ExpressionEvaluatingSelector selector = new ExpressionEvaluatingSelector(expression);
+			this.target.addRecipient(channel, selector);
+			this.componentsToRegister.add(selector);
+		}
+		else {
+			this.target.addRecipient(channel);
+		}
 		return _this();
 	}
 

spring-integration-core/src/test/java/org/springframework/integration/dsl/routers/RouterTests.java

@@ -61,6 +61,7 @@
 
 /**
  * @author Artem Bilan
+ * @author Gary Russell
  *
  * @since 5.0
  */
@@ -441,6 +442,10 @@ public void testPayloadTypeRouteFlow() {
 	@Qualifier("recipientListOrderResult")
 	private PollableChannel recipientListOrderResult;
 
+	@Autowired
+	@Qualifier("alwaysRecipient")
+	private QueueChannel alwaysRecipient;
+
 	@Test
 	@SuppressWarnings("unchecked")
 	public void testRecipientListRouterOrder() {
@@ -455,6 +460,8 @@ public void testRecipientListRouterOrder() {
 		assertNotNull(receive);
 		result = (AtomicReference<String>) receive.getPayload();
 		assertEquals("Hello World", result.get());
+
+		assertEquals(1, this.alwaysRecipient.getQueueSize());
 	}
 
 	@Autowired
@@ -582,7 +589,6 @@ public IntegrationFlow recipientListFlow() {
 					.get();
 		}
 
-
 		@Bean
 		public RoutingTestBean routingTestBean() {
 			return new RoutingTestBean();
@@ -646,6 +652,7 @@ public IntegrationFlow routerAsNonLastFlow() {
 		public IntegrationFlow recipientListOrderFlow() {
 			return f -> f
 					.routeToRecipients(r -> r
+							.recipient(alwaysRecipient())
 							.recipient("recipient2.input")
 							.recipient("recipient1.input"));
 		}
@@ -675,6 +682,11 @@ public PollableChannel recipientListOrderResult() {
 			return new QueueChannel();
 		}
 
+		@Bean
+		public QueueChannel alwaysRecipient() {
+			return new QueueChannel();
+		}
+
 		@Bean
 		public IntegrationFlow scatterGatherFlow() {
 			return f -> f