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`.
server/src/main/java/org/elasticsearch/action/support/replication/TransportBroadcastReplicationAction.java
TransportService transportService,
        NodeClient client,
        ActionFilters actionFilters,
        IndexNameExpressionResolver indexNameExpressionResolver,
        ActionType<ShardResponse> replicatedBroadcastShardAction
    ) {
        super(name, transportService, actionFilters, requestReader);
        this.client = client;
        this.replicatedBroadcastShardAction = replicatedBroadcastShardAction;
        this.clusterService = clusterService;
        this.indexNameExpressionResolver = indexNameExpressionResolver;
    }
    @Override
    protected void doExecute(Task task, Request request, ActionListener<Response> listener) {
        final ClusterState clusterState = clusterService.state();
        List<ShardId> shards = shards(request, clusterState);
        final CopyOnWriteArrayList<ShardResponse> shardsResponses = new CopyOnWriteArrayList<>();
        if (shards.size() == 0) {
            finishAndNotifyListener(listener, shardsResponses);
        }
        final CountDown responsesCountDown = new CountDown(shards.size());
        for (final ShardId shardId : shards) {
            ActionListener<ShardResponse> shardActionListener = new ActionListener<ShardResponse>() {
                @Override
                public void onResponse(ShardResponse shardResponse) {
                    shardsResponses.add(shardResponse);
                    logger.trace("{}: got response from {}", actionName, shardId);
                    if (responsesCountDown.countDown()) {
                        finishAndNotifyListener(listener, shardsResponses);
                    }