### 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
internalCluster().startNode(NodeRoles.onlyRole(DiscoveryNodeRole.DATA_HOT_NODE_ROLE));
        putAutoscalingPolicy("hot", DataTier.DATA_HOT);
        putAutoscalingPolicy("warm", DataTier.DATA_WARM);
        final String indexName = randomAlphaOfLength(10).toLowerCase(Locale.ROOT);
        assertAcked(
            prepareCreate(indexName).setSettings(
                Settings.builder()
                    .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
                    .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 6)
                    .put(INDEX_STORE_STATS_REFRESH_INTERVAL_SETTING.getKey(), "0ms")
                    .put(DataTier.TIER_PREFERENCE, allocatable ? "data_hot" : "data_content")
                    .build()
            ).setWaitForActiveShards(allocatable ? ActiveShardCount.DEFAULT : ActiveShardCount.NONE)
        );
        if (allocatable) {
            refresh();
        }
        assertThat(capacity().results().get("warm").requiredCapacity().total().storage().getBytes(), equalTo(0L));
        assertThat(capacity().results().get("warm").requiredCapacity().node().storage().getBytes(), equalTo(0L));
        assertAcked(
            client().admin()
                .indices()
                .updateSettings(
                    new UpdateSettingsRequest(indexName).settings(Settings.builder().put(DataTier.TIER_PREFERENCE, "data_warm,data_hot"))
                )
                .actionGet()
        );
        if (allocatable == false) {
            refresh();
        }