Fix NullPointerException when no subscriber for slow event (#3835)

common/src/main/java/com/alibaba/nacos/common/notify/DefaultSharePublisher.java

@@ -96,6 +96,10 @@ public class DefaultSharePublisher extends DefaultPublisher {
         
         // Get for Map, the algorithm is O(1).
         Set<Subscriber> subscribers = subMappings.get(slowEventType);
+        if (null == subscribers) {
+            LOGGER.debug("[NotifyCenter] No subscribers for slow event {}", slowEventType.getName());
+            return;
+        }
         
         // Notification single event subscriber
         for (Subscriber subscriber : subscribers) {

common/src/test/java/com/alibaba/nacos/common/notify/NotifyCenterTest.java

@@ -132,6 +132,7 @@ public class NotifyCenterTest {
                 return ExpireEvent.class;
             }
             
+            @Override
             public boolean ignoreExpireEvent() {
                 return true;
             }