### 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
import org.elasticsearch.cluster.routing.allocation.DataTier;
import org.elasticsearch.common.settings.ClusterSettings;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.node.Node;
import org.elasticsearch.test.ESIntegTestCase;
import org.elasticsearch.test.NodeRoles;
import org.elasticsearch.test.junit.annotations.TestLogging;
import org.elasticsearch.transport.TransportService;
import org.elasticsearch.xpack.autoscaling.action.GetAutoscalingCapacityAction;
import org.elasticsearch.xpack.autoscaling.action.PutAutoscalingPolicyAction;
import org.hamcrest.Matchers;
import java.util.Arrays;
import java.util.Locale;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
import java.util.TreeSet;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import static org.elasticsearch.index.store.Store.INDEX_STORE_STATS_REFRESH_INTERVAL_SETTING;
import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.is;
import static org.hamcrest.Matchers.notNullValue;
@ESIntegTestCase.ClusterScope(scope = ESIntegTestCase.Scope.TEST, numDataNodes = 0)
public class ReactiveStorageIT extends AutoscalingStorageIntegTestCase {
    public void testScaleUp() throws InterruptedException {
        internalCluster().startMasterOnlyNode();
        final String dataNodeName = internalCluster().startDataOnlyNode();