`GET /_snapshot/_all/_all` times out when there are no repositories

[jen-huang]

Relates to changes in #42090

When there are no repositories, a request to GET /_snapshot/_all/_all returns a 504 timeout error.

I would expect this request to return [] (an empty array).

[elasticmachine]

Pinging @elastic/es-distributed

[original-brownbear]

@andrershov I think we just need to short-circuit the listener in org.elasticsearch.action.admin.cluster.snapshots.get.TransportGetSnapshotsAction#getMultipleReposSnapshotInfo, instantiating a group action listener with count 0 (illegal argument) seems to be killing us here.