Fix test infra NPE in doEnsureClusterStateConsistency (#131859)

Author: DiannaHohensee

test/framework/src/main/java/org/elasticsearch/test/ESIntegTestCase.java

@@ -1382,6 +1382,15 @@ private void setMultiProjectParams(Map<String, String> xContentParams) {
         }
     }
 
+    /**
+     * Verifies that all nodes in the cluster see the same master node and cluster UUID and use the same JSON serialization.
+     * <ul>
+     * <li> Fetches the cluster state from every node in the cluster </li>
+     * <li> Fetches the master node's view of the cluster state </li>
+     * <li> Compares each node's view with the master node's </li>
+     * </ul>
+     * @param namedWriteableRegistry
+     */
     protected final void doEnsureClusterStateConsistency(NamedWriteableRegistry namedWriteableRegistry) {
         // This check has very little value in external test clusters and there is no guaranteed method of obtaining the master cluster
         // state in those clusters.
@@ -1409,6 +1418,10 @@ protected final void doEnsureClusterStateConsistency(NamedWriteableRegistry name
                 );
                 Map<String, Object> masterStateMap = convertToMap(masterClusterState, xContentParams());
                 String masterId = masterClusterState.nodes().getMasterNodeId();
+                if (masterId == null) {
+                    logger.warn("Failed to find an elected master in the cluster state: " + masterClusterState);
+                    throw new AssertionError("Unable to find master in cluster state. Expecting a stable master node");
+                }
                 for (SubscribableListener<ClusterStateResponse> localStateListener : localStates) {
                     localStateListener.andThenAccept(localClusterStateResponse -> {
                         byte[] localClusterStateBytes = ClusterState.Builder.toBytes(localClusterStateResponse.getState());