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
if (request.shardStatuses().contains(shardHealth.getStatus())) {
                    shardsToFetch.add(Tuple.tuple(routing.shardId(), customDataPath));
                }
            }
        }
        
        
        
        
        
        new AsyncShardStoresInfoFetches(state.nodes(), routingNodes, shardsToFetch, listener).start();
    }
    @Override
    protected ClusterBlockException checkBlock(IndicesShardStoresRequest request, ClusterState state) {
        return state.blocks()
            .indicesBlockedException(ClusterBlockLevel.METADATA_READ, indexNameExpressionResolver.concreteIndexNames(state, request));
    }
    private class AsyncShardStoresInfoFetches {
        private final DiscoveryNodes nodes;
        private final RoutingNodes routingNodes;
        private final Set<Tuple<ShardId, String>> shards;
        private final ActionListener<IndicesShardStoresResponse> listener;
        private CountDown expectedOps;
        private final Queue<InternalAsyncFetch.Response> fetchResponses;
        AsyncShardStoresInfoFetches(
            DiscoveryNodes nodes,
            RoutingNodes routingNodes,
            Set<Tuple<ShardId, String>> shards,
            ActionListener<IndicesShardStoresResponse> listener
        ) {