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
InternalAsyncFetch(Logger logger, String type, ShardId shardId, String customDataPath) {
                super(logger, type, shardId, customDataPath);
            }
            @Override
            protected synchronized void processAsyncFetch(
                List<NodeGatewayStartedShards> responses,
                List<FailedNodeException> failures,
                long fetchingRound
            ) {
                fetchResponses.add(new Response(shardId, responses, failures));
                if (expectedOps.countDown()) {
                    finish();
                }
            }
            void finish() {
                Map<String, Map<Integer, List<StoreStatus>>> indicesStatuses = new HashMap<>();
                List<Failure> failures = new ArrayList<>();
                for (Response fetchResponse : fetchResponses) {
                    var indexName = fetchResponse.shardId.getIndexName();
                    var shardId = fetchResponse.shardId.id();
                    var indexStatuses = indicesStatuses.computeIfAbsent(indexName, k -> new HashMap<>());
                    var storeStatuses = indexStatuses.computeIfAbsent(shardId, k -> new ArrayList<>());
                    for (NodeGatewayStartedShards r : fetchResponse.responses) {
                        if (shardExistsInNode(r)) {
                            var allocationStatus = getAllocationStatus(indexName, shardId, r.getNode());
                            storeStatuses.add(new StoreStatus(r.getNode(), r.allocationId(), allocationStatus, r.storeException()));
                        }
                    }
                    for (FailedNodeException failure : fetchResponse.failures) {
                        failures.add(new Failure(failure.nodeId(), indexName, shardId, failure.getCause()));