### 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
final String policyName = "test";
        putAutoscalingPolicy(policyName, "data");
        final String indexName = randomAlphaOfLength(10).toLowerCase(Locale.ROOT);
        createIndex(
            indexName,
            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")
                .build()
        );
        indexRandom(
            true,
            IntStream.range(1, 100)
                .mapToObj(i -> client().prepareIndex(indexName).setSource("field", randomAlphaOfLength(50)))
                .toArray(IndexRequestBuilder[]::new)
        );
        forceMerge();
        refresh();
        
        capacity();
        IndicesStatsResponse stats = client().admin().indices().prepareStats(indexName).clear().setStore(true).get();
        long used = stats.getTotal().getStore().getSizeInBytes();
        long minShardSize = Arrays.stream(stats.getShards()).mapToLong(s -> s.getStats().getStore().sizeInBytes()).min().orElseThrow();
        long maxShardSize = Arrays.stream(stats.getShards()).mapToLong(s -> s.getStats().getStore().sizeInBytes()).max().orElseThrow();
        long enoughSpace = used + HIGH_WATERMARK_BYTES + 1;
        setTotalSpace(dataNodeName, enoughSpace);
        GetAutoscalingCapacityAction.Response response = capacity();
        assertThat(response.results().keySet(), equalTo(Set.of(policyName)));
        assertThat(response.results().get(policyName).currentCapacity().total().storage().getBytes(), equalTo(enoughSpace));