GH-1220: Fix Sonar Issue

Author: garyrussell

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/listener/RabbitListenerEndpoint.java

@@ -153,6 +153,7 @@ default ReplyPostProcessor getReplyPostProcessor() {
 	 * @return the content type.
 	 * @since 2.3
 	 */
+	@Nullable
 	default String getReplyContentType() {
 		return null;
 	}