Fix NPE in AdaptiveAllocationsScalarService for null nodes (serverless) (#124318)

Author: jan-elastic

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/adaptiveallocations/AdaptiveAllocationsScalerService.java

@@ -15,6 +15,7 @@
 import org.elasticsearch.cluster.ClusterChangedEvent;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.ClusterStateListener;
+import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.util.concurrent.EsRejectedExecutionException;
@@ -383,7 +384,8 @@ private void processDeploymentStats(GetDeploymentStatsAction.Response statsRespo
                 key -> new HashMap<>()
             );
             for (AssignmentStats.NodeStats nodeStats : assignmentStats.getNodeStats()) {
-                String nodeId = nodeStats.getNode().getId();
+                DiscoveryNode node = nodeStats.getNode();
+                String nodeId = node == null ? null : node.getId();
                 Stats lastStats = deploymentStats.get(nodeId);
                 Stats nextStats = new Stats(
                     nodeStats.getInferenceCount().orElse(0L),

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/inference/adaptiveallocations/AdaptiveAllocationsScalerServiceTests.java

@@ -130,7 +130,7 @@ private GetDeploymentStatsAction.Response getDeploymentStatsResponse(int numAllo
                     Instant.now(),
                     List.of(
                         AssignmentStats.NodeStats.forStartedState(
-                            DiscoveryNodeUtils.create("node_1"),
+                            randomBoolean() ? DiscoveryNodeUtils.create("node_1") : null,
                             inferenceCount,
                             latency,
                             latency,