### 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
assertThat(response.results().get(policyName).requiredCapacity().total().storage().getBytes(), equalTo(enoughSpace));
        assertThat(
            response.results().get(policyName).requiredCapacity().node().storage().getBytes(),
            equalTo(maxShardSize + ReactiveStorageDeciderService.NODE_DISK_OVERHEAD + LOW_WATERMARK_BYTES)
        );
        setTotalSpace(dataNodeName, enoughSpace - 2);
        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(),
            Matchers.greaterThan(enoughSpace - 2)
        );
        assertThat(
            response.results().get(policyName).requiredCapacity().total().storage().getBytes(),
            Matchers.lessThanOrEqualTo(enoughSpace + minShardSize)
        );
        assertThat(
            response.results().get(policyName).requiredCapacity().node().storage().getBytes(),
            equalTo(maxShardSize + ReactiveStorageDeciderService.NODE_DISK_OVERHEAD + LOW_WATERMARK_BYTES)
        );
    }
    public void testScaleFromEmptyWarmMove() throws Exception {
        testScaleFromEmptyWarm(true);
    }
    public void testScaleFromEmptyWarmUnassigned() throws Exception {
        testScaleFromEmptyWarm(false);
    }
    private void testScaleFromEmptyWarm(boolean allocatable) throws Exception {
        internalCluster().startMasterOnlyNode();