Fix NPE for @Indexed in the MessagingGatewayReg

The `@Indexed` comes without any attributes, but it is included
into a chain via `importingClassMetadata.getMetaAnnotationTypes(ann)`.
So, skip attributes from meta annotation when it is `null`

**Cherry-pick to `5.4.x` & `5.3.x`**

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/annotation/MessagingGateway.java

@@ -45,7 +45,6 @@
  */
 @Target(ElementType.TYPE)
 @Retention(RetentionPolicy.RUNTIME)
-@Documented
 @Indexed
 public @interface MessagingGateway {
 

spring-integration-core/src/main/java/org/springframework/integration/config/MessagingGatewayRegistrar.java

@@ -199,7 +199,10 @@ private static List<MultiValueMap<String, Object>> captureMetaAnnotationValues(
 			Set<String> chain = importingClassMetadata.getMetaAnnotationTypes(ann);
 			if (chain.contains(MessagingGateway.class.getName())) {
 				for (String meta : chain) {
-					valuesHierarchy.add(importingClassMetadata.getAllAnnotationAttributes(meta));
+					MultiValueMap<String, Object> attributes = importingClassMetadata.getAllAnnotationAttributes(meta);
+					if (attributes != null) {
+						valuesHierarchy.add(attributes);
+					}
 				}
 			}
 		}