Today in a few places we create a `new CountDown(0)`. This is kind of trappy because it will never return `true` from `#countDown()` or `#fastForward()` which risks never completing whatever action needs completing unless the caller handles the empty case specially.

We should refactor the places that do this so that they all avoid creating an empty `CountDown`, instead just handling the fact that the action is already complete:

- [ ] `TransportIndicesShardStoresAction$AsyncShardStoresInfoFetches`
- [ ] `ClearScrollController`
- [ ] `TransportBroadcastReplicationAction#doExecute`
- [ ] `Realms#usageStats`

Once we've done that, we can forbid the creation of an empty `CountDown`.
x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/Realms.java
public void usageStats(ActionListener<Map<String, Object>> listener) {
        final XPackLicenseState licenseStateSnapshot = licenseState.copyCurrentLicenseState();
        Map<String, Object> realmMap = new HashMap<>();
        final AtomicBoolean failed = new AtomicBoolean(false);
        final List<Realm> realmList = getActiveRealms().stream().filter(r -> ReservedRealm.TYPE.equals(r.type()) == false).toList();
        final CountDown countDown = new CountDown(realmList.size());
        final Runnable doCountDown = () -> {
            if ((realmList.isEmpty() || countDown.countDown()) && failed.get() == false) {
                
                for (String type : factories.keySet()) {
                    assert ReservedRealm.TYPE.equals(type) == false;
                    realmMap.compute(type, (key, value) -> {
                        if (value == null) {
                            return MapBuilder.<String, Object>newMapBuilder()
                                .put("enabled", false)
                                .put("available", isRealmTypeAvailable(licenseStateSnapshot, type))
                                .map();
                        }
                        assert value instanceof Map;
                        @SuppressWarnings("unchecked")
                        Map<String, Object> realmTypeUsage = (Map<String, Object>) value;
                        realmTypeUsage.put("enabled", true);
                        realmTypeUsage.put("available", true);
                        return value;
                    });
                }
                listener.onResponse(realmMap);
            }
        };
        if (realmList.isEmpty()) {