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
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.action.admin.indices.shards;
import org.elasticsearch.ElasticsearchException;
import org.elasticsearch.action.ActionResponse;
import org.elasticsearch.action.support.DefaultShardOperationFailedException;
import org.elasticsearch.cluster.node.DiscoveryNode;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.common.io.stream.Writeable;
import org.elasticsearch.xcontent.ToXContentFragment;
import org.elasticsearch.xcontent.XContentBuilder;
import java.io.IOException;
import java.util.Collections;
import java.util.List;
import java.util.Map;
/**
 * Response for {@link IndicesShardStoresAction}
 *
 * Consists of {@link StoreStatus}s for requested indices grouped by
 * indices and shard ids and a list of encountered node {@link Failure}s
 */
public class IndicesShardStoresResponse extends ActionResponse implements ToXContentFragment {