### CI Link

https://gradle-enterprise.elastic.co/s/opv3672lmdhmi

### Repro line

./gradlew ':x-pack:plugin:autoscaling:internalClusterTest' --tests "org.elasticsearch.xpack.autoscaling.storage.ReactiveStorageIT.testScaleWhileShrinking" -Dtests.seed=F89DAF53966040FE -Dtests.locale=ar-BH -Dtests.timezone=Europe/Gibraltar -Druntime.java=19

### Does it reproduce?

No

### Applicable branches

main

### Failure history

https://gradle-enterprise.elastic.co/scans/tests?search.timeZoneId=Europe/Rome&tests.container=org.elasticsearch.xpack.autoscaling.storage.ReactiveStorageIT&tests.test=testScaleWhileShrinking

### Failure excerpt

```
java.lang.NullPointerException: Cannot invoke "org.elasticsearch.cluster.DiskUsage.getFreeBytes()" because the return value of "java.util.Map.get(Object)" is null
```

It looks like method `getNodeMostAvailableDiskUsages` returns a null value which results in a NPE when calling get on the expected result map.
x-pack/plugin/autoscaling/src/internalClusterTest/java/org/elasticsearch/xpack/autoscaling/storage/ReactiveStorageIT.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.autoscaling.storage;
import org.elasticsearch.action.admin.indices.settings.put.UpdateSettingsRequest;
import org.elasticsearch.action.admin.indices.shrink.ResizeType;
import org.elasticsearch.action.admin.indices.stats.IndicesStatsResponse;
import org.elasticsearch.action.index.IndexRequestBuilder;
import org.elasticsearch.action.support.ActiveShardCount;
import org.elasticsearch.cluster.ClusterInfo;
import org.elasticsearch.cluster.ClusterInfoService;
import org.elasticsearch.cluster.metadata.IndexMetadata;
import org.elasticsearch.cluster.node.DiscoveryNodeRole;