### 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
.collect(Collectors.toList())
            )
        );
    }
    public void setTotalSpace(String dataNodeName, long totalSpace) {
        getTestFileStore(dataNodeName).setTotalSpace(totalSpace);
        refreshClusterInfo();
    }
    public GetAutoscalingCapacityAction.Response capacity() {
        GetAutoscalingCapacityAction.Request request = new GetAutoscalingCapacityAction.Request();
        GetAutoscalingCapacityAction.Response response = client().execute(GetAutoscalingCapacityAction.INSTANCE, request).actionGet();
        return response;
    }
    private void putAutoscalingPolicy(String policyName, String role) {
        final PutAutoscalingPolicyAction.Request request = new PutAutoscalingPolicyAction.Request(
            policyName,
            new TreeSet<>(Set.of(role)),
            new TreeMap<>(Map.of("reactive_storage", Settings.EMPTY))
        );
        assertAcked(client().execute(PutAutoscalingPolicyAction.INSTANCE, request).actionGet());
    }
    private ClusterInfo getClusterInfo() {
        final ClusterInfoService clusterInfoService = internalCluster().getInstance(
            ClusterInfoService.class,
            internalCluster().getMasterName()
        );
        return clusterInfoService.getClusterInfo();
    }
}