### 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
assertAcked(client().admin().cluster().prepareReroute());
        ensureGreen();
        client().admin().indices().prepareDelete(indexName).get();
        response = capacity();
        assertThat(
            response.results().get(policyName).requiredCapacity().total().storage().getBytes(),
            equalTo(requiredSpaceForClone + enoughSpace)
        );
    }
    /**
     * Verify that the list of roles includes all data roles except frozen to ensure we consider adding future data roles.
     */
    public void testRoles() {
        
        internalCluster().startMasterOnlyNode();
        ReactiveStorageDeciderService service = new ReactiveStorageDeciderService(
            Settings.EMPTY,
            new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS),
            null
        );
        assertThat(
            service.roles().stream().sorted().collect(Collectors.toList()),
            equalTo(
                DiscoveryNodeRole.roles()
                    .stream()
                    .filter(DiscoveryNodeRole::canContainData)
                    .filter(r -> r != DiscoveryNodeRole.DATA_FROZEN_NODE_ROLE)
                    .filter(r -> r != DiscoveryNodeRole.SEARCH_ROLE)
                    .filter(r -> r != DiscoveryNodeRole.INDEX_ROLE)
                    .sorted()