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
builder.endObject();
            }
            return builder;
        }
        @Override
        public int compareTo(StoreStatus other) {
            if (storeException != null && other.storeException == null) {
                return 1;
            } else if (other.storeException != null && storeException == null) {
                return -1;
            }
            if (allocationId != null && other.allocationId == null) {
                return -1;
            } else if (allocationId == null && other.allocationId != null) {
                return 1;
            } else if (allocationId == null && other.allocationId == null) {
                return Integer.compare(allocationStatus.id, other.allocationStatus.id);
            } else {
                int compare = Integer.compare(allocationStatus.id, other.allocationStatus.id);
                if (compare == 0) {
                    return allocationId.compareTo(other.allocationId);
                }
                return compare;
            }
        }
    }
    /**
     * Single node failure while retrieving shard store information
     */
    public static class Failure extends DefaultShardOperationFailedException {