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/IndicesShardStoresResponse.java
return storeStatuses;
    }
    /**
     * Returns node {@link Failure}s encountered
     * while executing the request
     */
    public List<Failure> getFailures() {
        return failures;
    }
    @Override
    public void writeTo(StreamOutput out) throws IOException {
        out.writeMap(
            storeStatuses,
            StreamOutput::writeString,
            (o, v) -> o.writeMap(v, StreamOutput::writeInt, StreamOutput::writeCollection)
        );
        out.writeList(failures);
    }
    @Override
    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
        if (failures.size() > 0) {
            builder.startArray(Fields.FAILURES);
            for (Failure failure : failures) {
                failure.toXContent(builder, params);
            }
            builder.endArray();
        }
        builder.startObject(Fields.INDICES);
        for (Map.Entry<String, Map<Integer, List<StoreStatus>>> indexShards : storeStatuses.entrySet()) {
            builder.startObject(indexShards.getKey());