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
return domainNameToConfig.entrySet()
                .stream()
                .collect(Collectors.toMap(Entry::getKey, entry -> Map.of("realms", entry.getValue().memberRealmNames())));
        }
    }
    public Map<RealmConfig.RealmIdentifier, Authentication.RealmRef> getRealmRefs() {
        return realmRefs;
    }
    @Override
    protected void doStart() {}
    @Override
    protected void doStop() {}
    @Override
    protected void doClose() throws IOException {
        IOUtils.close(allConfiguredRealms.stream().filter(r -> r instanceof Closeable).map(r -> (Closeable) r).toList());
    }
    private void maybeAddBasicRealms(List<Realm> realms, List<RealmConfig> realmConfigs) throws Exception {
        final Set<String> disabledBasicRealmTypes = findDisabledBasicRealmTypes(realmConfigs);
        final Set<String> realmTypes = realms.stream().map(Realm::type).collect(Collectors.toUnmodifiableSet());
        
        if (false == disabledBasicRealmTypes.contains(NativeRealmSettings.TYPE) && false == realmTypes.contains(NativeRealmSettings.TYPE)) {
            ensureRealmNameIsAvailable(realms, NativeRealmSettings.DEFAULT_NAME);
            var nativeRealmId = new RealmConfig.RealmIdentifier(NativeRealmSettings.TYPE, NativeRealmSettings.DEFAULT_NAME);
            var realmConfig = new RealmConfig(
                nativeRealmId,
                ensureOrderSetting(settings, nativeRealmId, Integer.MIN_VALUE),
                env,
                threadContext
            );
            realmConfigs.add(realmConfig);