### Elasticsearch Version

8.2.0 (likely others)

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

Cloud

### Problem Description

I saw a NPE in `MlAssignmentNotifier#auditMlTasks` which looks to be due to the `PersistentTasksCustomMetadata` being completely missing (i.e. `null`) rather than simply empty.

### Steps to Reproduce

Unknown

### Logs (if relevant)

```
[instance-0000000000] uncaught exception in thread [elasticsearch[instance-0000000000][generic][T#5]]
java.lang.NullPointerException: Cannot invoke "org.elasticsearch.persistent.PersistentTasksCustomMetadata.tasks()" because "currentTasks" is null
	at org.elasticsearch.xpack.ml.MlAssignmentNotifier.auditMlTasks(MlAssignmentNotifier.java:96) ~[?:?]
	at org.elasticsearch.xpack.ml.MlAssignmentNotifier.auditChangesToMlTasks(MlAssignmentNotifier.java:76) ~[?:?]
	at org.elasticsearch.xpack.ml.MlAssignmentNotifier.lambda$clusterChanged$0(MlAssignmentNotifier.java:64) ~[?:?]
	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:714) ~[elasticsearch-8.2.0.jar:8.2.0]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[?:?]
	at java.lang.Thread.run(Thread.java:833) [?:?]
```
x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/MlAssignmentNotifier.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.ml;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.elasticsearch.cluster.ClusterChangedEvent;
import org.elasticsearch.cluster.ClusterStateListener;
import org.elasticsearch.cluster.node.DiscoveryNode;
import org.elasticsearch.cluster.node.DiscoveryNodes;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.Strings;
import org.elasticsearch.persistent.PersistentTasksCustomMetadata;
import org.elasticsearch.persistent.PersistentTasksCustomMetadata.Assignment;
import org.elasticsearch.persistent.PersistentTasksCustomMetadata.PersistentTask;
import org.elasticsearch.threadpool.ThreadPool;
import org.elasticsearch.xpack.core.ml.MlTasks;
import org.elasticsearch.xpack.core.ml.action.OpenJobAction;
import org.elasticsearch.xpack.core.ml.action.StartDataFrameAnalyticsAction;
import org.elasticsearch.xpack.core.ml.action.StartDatafeedAction;
import org.elasticsearch.xpack.ml.notifications.AnomalyDetectionAuditor;
import org.elasticsearch.xpack.ml.notifications.DataFrameAnalyticsAuditor;
import java.util.Objects;
public class MlAssignmentNotifier implements ClusterStateListener {