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
this.id = id;
            }
            private static AllocationStatus fromId(byte id) {
                return switch (id) {
                    case 0 -> PRIMARY;
                    case 1 -> REPLICA;
                    case 2 -> UNUSED;
                    default -> throw new IllegalArgumentException("unknown id for allocation status [" + id + "]");
                };
            }
            public String value() {
                return switch (id) {
                    case 0 -> "primary";
                    case 1 -> "replica";
                    case 2 -> "unused";
                    default -> throw new IllegalArgumentException("unknown id for allocation status [" + id + "]");
                };
            }
            private static AllocationStatus readFrom(StreamInput in) throws IOException {
                return fromId(in.readByte());
            }
            private void writeTo(StreamOutput out) throws IOException {
                out.writeByte(id);
            }
        }
        public StoreStatus(StreamInput in) throws IOException {
            node = new DiscoveryNode(in);
            allocationId = in.readOptionalString();
            allocationStatus = AllocationStatus.readFrom(in);
            if (in.readBoolean()) {