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/search/ClearScrollController.java
if (contexts.isEmpty()) {
                expectedOps = 0;
                runner = () -> listener.onResponse(new ClearScrollResponse(true, 0));
            } else {
                expectedOps = contexts.size();
                runner = () -> cleanScrollIds(contexts);
            }
        }
        this.expectedOps = new CountDown(expectedOps);
    }
    @Override
    public void run() {
        runner.run();
    }
    void cleanAllScrolls() {
        for (final DiscoveryNode node : nodes) {
            try {
                Transport.Connection connection = searchTransportService.getConnection(null, node);
                searchTransportService.sendClearAllScrollContexts(connection, new ActionListener<TransportResponse>() {
                    @Override
                    public void onResponse(TransportResponse response) {
                        onFreedContext(true);
                    }
                    @Override
                    public void onFailure(Exception e) {
                        onFailedFreedContext(e, node);
                    }
                });
            } catch (Exception e) {
                onFailedFreedContext(e, node);