### 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
case 0 -> Tuple.tuple("_id", dataNode1Id);
            case 1 -> Tuple.tuple("_name", dataNode1Name);
            case 2 -> Tuple.tuple("name", dataNode1Name);
            default -> throw new IllegalArgumentException();
        };
        String filterKey = randomFrom(IndexMetadata.INDEX_ROUTING_INCLUDE_GROUP_SETTING, IndexMetadata.INDEX_ROUTING_REQUIRE_GROUP_SETTING)
            .getKey() + filter.v1();
        assertAcked(
            client().admin()
                .indices()
                .updateSettings(
                    new UpdateSettingsRequest(indexName).settings(
                        Settings.builder().put(filterKey, filter.v2()).put("index.blocks.write", true)
                    )
                )
                .actionGet()
        );
        long shrinkSpace = used + LOW_WATERMARK_BYTES;
        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(shrinkSpace + ReactiveStorageDeciderService.NODE_DISK_OVERHEAD)
        );
        long enoughSpaceForColocation = used + LOW_WATERMARK_BYTES;
        setTotalSpace(dataNode1Name, enoughSpaceForColocation);
        setTotalSpace(dataNode2Name, enoughSpaceForColocation);
        assertAcked(client().admin().cluster().prepareReroute());