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/admin/indices/shards/TransportIndicesShardStoresAction.java
IndicesShardStoresRequest::new,
            indexNameExpressionResolver,
            IndicesShardStoresResponse::new,
            ThreadPool.Names.SAME
        );
        this.client = client;
    }
    @Override
    protected void masterOperation(
        Task task,
        IndicesShardStoresRequest request,
        ClusterState state,
        ActionListener<IndicesShardStoresResponse> listener
    ) {
        final RoutingTable routingTables = state.routingTable();
        final RoutingNodes routingNodes = state.getRoutingNodes();
        final String[] concreteIndices = indexNameExpressionResolver.concreteIndexNames(state, request);
        final Set<Tuple<ShardId, String>> shardsToFetch = new HashSet<>();
        logger.trace("using cluster state version [{}] to determine shards", state.version());
        
        for (String index : concreteIndices) {
            IndexRoutingTable indexShardRoutingTables = routingTables.index(index);
            if (indexShardRoutingTables == null) {
                continue;
            }
            final String customDataPath = IndexMetadata.INDEX_DATA_PATH_SETTING.get(state.metadata().index(index).getSettings());
            for (int i = 0; i < indexShardRoutingTables.size(); i++) {
                IndexShardRoutingTable routing = indexShardRoutingTables.shard(i);
                final int shardId = routing.shardId().id();
                ClusterShardHealth shardHealth = new ClusterShardHealth(shardId, routing);