### 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
assertBusy(() -> {
            GetAutoscalingCapacityAction.Response response = capacity();
            assertThat(response.results().keySet(), equalTo(Set.of(policyName)));
            assertThat(response.results().get(policyName).requiredCapacity(), is(notNullValue()));
        });
        
        GetAutoscalingCapacityAction.Response response = capacity();
        assertThat(response.results().keySet(), equalTo(Set.of(policyName)));
        assertThat(response.results().get(policyName).currentCapacity().total().storage().getBytes(), equalTo(enoughSpace * 2));
        assertThat(response.results().get(policyName).requiredCapacity().total().storage().getBytes(), equalTo(enoughSpace * 2));
        assertThat(
            response.results().get(policyName).requiredCapacity().node().storage().getBytes(),
            equalTo(used + LOW_WATERMARK_BYTES + ReactiveStorageDeciderService.NODE_DISK_OVERHEAD)
        );
        assertAcked(
            client().admin()
                .indices()
                .updateSettings(new UpdateSettingsRequest(indexName).settings(Settings.builder().put("index.blocks.write", true)))
                .actionGet()
        );
        ResizeType resizeType = randomFrom(ResizeType.CLONE, ResizeType.SPLIT);
        String cloneName = "clone-" + indexName;
        int resizedShardCount = resizeType == ResizeType.CLONE ? 1 : between(2, 10);
        assertAcked(
            client().admin()
                .indices()
                .prepareResizeIndex(indexName, cloneName)
                .setSettings(