xinfo-groups: support nil lag in XINFO GROUPS (#3369)

* xinfo-groups: support nil lag in XINFO GROUPS

* Add test

* docs: clarify XInfoGroup.Lag field behavior with Nil values

* docs: clarify XInfoGroup.Lag field behavior

command.go

@@ -2104,7 +2104,9 @@ type XInfoGroup struct {
 	Pending         int64
 	LastDeliveredID string
 	EntriesRead     int64
-	Lag             int64
+	// Lag represents the number of pending messages in the stream not yet
+	// delivered to this consumer group. Returns -1 when the lag cannot be determined.
+	Lag int64
 }
 
 var _ Cmder = (*XInfoGroupsCmd)(nil)
@@ -2187,8 +2189,11 @@ func (cmd *XInfoGroupsCmd) readReply(rd *proto.Reader) error {
 
 				// lag: the number of entries in the stream that are still waiting to be delivered
 				// to the group's consumers, or a NULL(Nil) when that number can't be determined.
+				// In that case, we return -1.
 				if err != nil && err != Nil {
 					return err
+				} else if err == Nil {
+					group.Lag = -1
 				}
 			default:
 				return fmt.Errorf("redis: unexpected key %q in XINFO GROUPS reply", key)