[v10.4.x] Loki: Fix null pointer exception in case request returned an error (#84401)

Loki: Fix null pointer exception in case request returned an error (#84398)

Loki: Fix nullpointer in case query returned an error
(cherry picked from commit 34f9bff9e0)

Co-authored-by: Sven Grossmann <sven.grossmann@grafana.com>

pkg/tsdb/loki/loki.go

@@ -240,6 +240,11 @@ func executeQuery(ctx context.Context, query *lokiQuery, req *backend.QueryDataR
 	defer span.End()
 
 	queryRes, err := runQuery(ctx, api, query, responseOpts, plog)
+	if queryRes == nil {
+		// we always want to return a backend.DataResponse object, even if we received just an error
+		queryRes = &backend.DataResponse{}
+	}
+
 	if err != nil {
 		span.RecordError(err)
 		span.SetStatus(codes.Error, err.Error())