rpk group offset-delete: fix two bugs

* We should only request metadata for empty topics if we actually have
  empty topics. Requesting metadata with no empty topics requests
  metadata for the entire cluster, which results in offset-delete for
  all topics.
* For parsing topic partition args, we need to `make` with 0 length.
  Accidentally using the capacity as length results in issuing metadata
  requests with empty topic names (which cropped up another bug in kadm,
  which will be fixed by a dep bump before the next release)

src/go/rpk/pkg/cli/group/offset_delete.go

@@ -71,9 +71,11 @@ topic_b 0
 
  // For -topic options that didn't include partitions, perform a
  // lookup for them using a metadata request
- mdResp, err := adm.Metadata(context.Background(), topicsSet.EmptyTopics()...)
- out.MaybeDie(err, "unable to make metadata request: %v", err)
- topicsSet.Merge(mdResp.Topics.TopicsSet())
+ if empty := topicsSet.EmptyTopics(); len(empty) > 0 {
+ mdResp, err := adm.Metadata(context.Background(), empty...)
+ out.MaybeDie(err, "unable to make metadata request: %v", err)
+ topicsSet.Merge(mdResp.Topics.TopicsSet())
+ }
 
  responses, err := adm.DeleteOffsets(context.Background(), args[0], topicsSet)
  out.MaybeDieErr(err)
@@ -100,7 +102,7 @@ topic_b 0
 func parseTopicPartitionsArgs(list []string) (kadm.TopicsSet, error) {
  parsed, err := out.ParseTopicPartitions(list)
  if err == nil {
- topicsList := make(kadm.TopicsList, len(parsed))
+ topicsList := make(kadm.TopicsList, 0, len(parsed))
  for topic, partitions := range parsed {
  topicsList = append(topicsList, kadm.TopicPartitions{Topic: topic, Partitions: partitions})
  }