kgo: ensure we always have at least one shard

Non sharded requests always have one shard; that was a given before.
Sharded requests could avoid having any shards if the request actually
does not need to be issued. Rather than avoiding returning no shards,
this issues the request to any broker. This is not detrimental and makes
it easier to always know that there is at least one shard.

pkg/kgo/client.go

@@ -542,7 +542,9 @@ type ResponseShard struct {
 
 // RequestSharded performs the same logic as Request, but returns all responses
 // from any broker that the request was split to. This always returns at least
-// one shard.
+// one shard. If the request does not need to be issued (describing no groups),
+// this issues the request to a random broker just to ensure that one shard
+// exists.
 //
 // There are only a few requests that are strongly recommended to explicitly
 // use RequestSharded; the rest can by default use Request. These few requests
@@ -1214,6 +1216,17 @@ func (cl *Client) handleShardedReq(ctx context.Context, req kmsg.Request) ([]Res
 			return
 		}
 
+		// If the request actually does not need to be issued, we issue
+		// it to a random broker. There is no benefit to this, but at
+		// least we will return one shard.
+		if len(issues) == 0 {
+			issues = []issueShard{{
+				req: try.req,
+				any: true,
+			}}
+			reshardable = true
+		}
+
 		for i := range issues {
 			tries := try.tries
 			myIssue := issues[i]