kgo: allow empty groups when finding coordinator / fetching offsets

v1.10.0 introduced a bug when switching to batch loadCoordinators:
we would strip empty coordinator keys from the request.

Technically, empty coordinator keys are valid. If a request contains an
empty coordinator key (empty group), we would not load it and not set a
map key for it and then using a field in the map value would panic.

Now, we always request all coordinator keys.

This also showed a bug in sharded OffsetFetch: fetching offsets for a
group with no name would be stripped (now we will just forward whatever
error happens).

Closes #283.

pkg/kgo/client.go

@@ -1154,9 +1154,7 @@ func (cl *Client) loadCoordinators(ctx context.Context, typ int8, keys ...string
 
 	toRequest := make(map[string]bool, len(keys)) // true == bypass the cache
 	for _, key := range keys {
-		if len(key) > 0 {
-			toRequest[key] = false
-		}
+		toRequest[key] = false
 	}
 
 	// For each of these keys, we have two cases:
@@ -2324,9 +2322,7 @@ func (cl *offsetFetchSharder) shard(ctx context.Context, kreq kmsg.Request, last
 	}
 	groups := make([]string, 0, len(req.Groups))
 	for i := range req.Groups {
-		if g := req.Groups[i].Group; len(g) > 0 {
-			groups = append(groups, req.Groups[i].Group)
-		}
+		groups = append(groups, req.Groups[i].Group)
 	}
 
 	coordinators := cl.loadCoordinators(ctx, coordinatorTypeGroup, groups...)
@@ -2487,9 +2483,7 @@ func (*findCoordinatorSharder) shard(_ context.Context, kreq kmsg.Request, lastE
 	}
 	req.CoordinatorKeys = req.CoordinatorKeys[:0]
 	for key := range uniq {
-		if len(key) > 0 {
-			req.CoordinatorKeys = append(req.CoordinatorKeys, key)
-		}
+		req.CoordinatorKeys = append(req.CoordinatorKeys, key)
 	}
 
 	splitReq := errors.Is(lastErr, errBrokerTooOld)