Fix NPE in NodeSeenService (#76788) (#76796)

This commit adjusts `NodeSeenService` to check if the `NodesShutdownMetadata`
is `null` before trying to work with it. If it is `null`, we can simply
return from the method early as we know there are no shutdowns registered.

x-pack/plugin/shutdown/src/main/java/org/elasticsearch/xpack/shutdown/NodeSeenService.java

@@ -52,6 +52,12 @@ public void clusterChanged(ClusterChangedEvent event) {
         }
 
         NodesShutdownMetadata eventShutdownMetadata = event.state().metadata().custom(NodesShutdownMetadata.TYPE);
+
+        if (eventShutdownMetadata == null) {
+            // Since there's no shutdown metadata at all, we know no shutdowns have ever been registered and we can bail.
+            return;
+        }
+
         final Set<String> nodesNotPreviouslySeen = eventShutdownMetadata.getAllNodeMetadataMap()
             .values()
             .stream()