fix(proto): skip empty UserData during plan

This avoids a subsequent decoder failure where UserData is empty

Signed-off-by: Dominic Evans <dominic.evans@uk.ibm.com>

balance_strategy.go

@@ -967,6 +967,9 @@ func prepopulateCurrentAssignments(members map[string]ConsumerGroupMemberMetadat
 	// for each partition we create a sorted map of its consumers by generation
 	sortedPartitionConsumersByGeneration := make(map[topicPartitionAssignment]map[int]string)
 	for memberID, meta := range members {
+		if len(meta.UserData) == 0 {
+			continue
+		}
 		consumerUserData, err := deserializeTopicPartitionAssignment(meta.UserData)
 		if err != nil {
 			return nil, nil, err