rpcserver: improve error handling when querying for asset group given ID

This commit adds code which handles `err != nil` which was previously
absent.

Author: ffranr

rpcserver.go

@@ -2092,8 +2092,18 @@ func (r *rpcServer) BurnAsset(ctx context.Context,
 
 	var groupKey *btcec.PublicKey
 	assetGroup, err := r.cfg.TapAddrBook.QueryAssetGroup(ctx, assetID)
-	if err == nil && assetGroup.GroupKey != nil {
+	switch {
+	case err == nil && assetGroup.GroupKey != nil:
+		// We found the asset group, so we can use the group key to
+		// burn the asset.
 		groupKey = &assetGroup.GroupPubKey
+	case errors.Is(err, address.ErrAssetGroupUnknown):
+		// We don't know the asset group, so we'll try to burn the
+		// asset using the asset ID only.
+		rpcsLog.Debug("Asset group key not found, asset may not be " +
+			"part of a group")
+	case err != nil:
+		return nil, fmt.Errorf("error querying asset group: %w", err)
 	}
 
 	fundResp, err := r.cfg.AssetWallet.FundBurn(