### 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
IntStream.range(1, 100)
                .mapToObj(i -> client().prepareIndex(indexName).setSource("field", randomAlphaOfLength(50)))
                .toArray(IndexRequestBuilder[]::new)
        );
        forceMerge();
        refresh();
        IndicesStatsResponse stats = client().admin().indices().prepareStats(indexName).clear().setStore(true).get();
        long used = stats.getTotal().getStore().getSizeInBytes();
        long maxShardSize = Arrays.stream(stats.getShards()).mapToLong(s -> s.getStats().getStore().sizeInBytes()).max().orElseThrow();
        Map<String, Long> byNode = Arrays.stream(stats.getShards())
            .collect(
                Collectors.groupingBy(
                    s -> s.getShardRouting().currentNodeId(),
                    Collectors.summingLong(s -> s.getStats().getStore().getSizeInBytes())
                )
            );
        long enoughSpace1 = byNode.get(dataNode1Id).longValue() + HIGH_WATERMARK_BYTES + 1;
        long enoughSpace2 = byNode.get(dataNode2Id).longValue() + HIGH_WATERMARK_BYTES + 1;
        long enoughSpace = enoughSpace1 + enoughSpace2;
        setTotalSpace(dataNode1Name, enoughSpace1);
        setTotalSpace(dataNode2Name, enoughSpace2);
        GetAutoscalingCapacityAction.Response response = capacity();
        assertThat(response.results().keySet(), equalTo(Set.of(policyName)));
        assertThat(response.results().get(policyName).currentCapacity().total().storage().getBytes(), equalTo(enoughSpace));
        assertThat(response.results().get(policyName).requiredCapacity().total().storage().getBytes(), equalTo(enoughSpace));
        assertThat(
            response.results().get(policyName).requiredCapacity().node().storage().getBytes(),
            equalTo(maxShardSize + LOW_WATERMARK_BYTES + ReactiveStorageDeciderService.NODE_DISK_OVERHEAD)
        );
        Tuple<String, String> filter = switch (between(0, 2)) {