ebs br: fail backup when no tikv store (#53783)

close #53784

br/pkg/task/backup_ebs.go

@@ -26,6 +26,7 @@ import (
 	"github.com/pingcap/tidb/br/pkg/config"
 	"github.com/pingcap/tidb/br/pkg/conn"
 	"github.com/pingcap/tidb/br/pkg/conn/util"
+	berrors "github.com/pingcap/tidb/br/pkg/errors"
 	"github.com/pingcap/tidb/br/pkg/glue"
 	"github.com/pingcap/tidb/br/pkg/metautil"
 	"github.com/pingcap/tidb/br/pkg/pdutil"
@@ -100,7 +101,7 @@ func RunBackupEBS(c context.Context, g glue.Glue, cfg *BackupConfig) error {
 	storeCount := backupInfo.GetStoreCount()
 	if storeCount == 0 {
 		log.Info("nothing to backup")
-		return nil
+		return errors.Trace(errors.Annotate(berrors.ErrInvalidArgument, "store count is 0"))
 	}
 
 	if span := opentracing.SpanFromContext(ctx); span != nil && span.Tracer() != nil {