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/common/util/concurrent/CountDown.java
/**
     * Decrements the count-down and returns <code>true</code> iff this call
     * reached zero otherwise <code>false</code>
     */
    public boolean countDown() {
        assert originalCount > 0;
        return countDown.getAndUpdate(current -> {
            assert current >= 0;
            return current == 0 ? 0 : current - 1;
        }) == 1;
    }
    /**
     * Fast forwards the count-down to zero and returns <code>true</code> iff
     * the count down reached zero with this fast forward call otherwise
     * <code>false</code>
     */
    public boolean fastForward() {
        assert originalCount > 0;
        assert countDown.get() >= 0;
        return countDown.getAndSet(0) > 0;
    }
    /**
     * Returns <code>true</code> iff the count-down has reached zero. Otherwise <code>false</code>
     */
    public boolean isCountedDown() {
        assert countDown.get() >= 0;
        return countDown.get() == 0;
    }
}