Dashboard: No longer includes default datasource when externally exporting dashboard with row (#32494)

Closes #31065

public/app/features/dashboard/components/DashExportModal/DashboardExporter.ts

@@ -109,7 +109,7 @@ export class DashboardExporter {
     };
 
     const processPanel = (panel: PanelModel) => {
-      if (panel.datasource !== undefined) {
+      if (panel.datasource !== undefined && panel.datasource !== null) {
         templateizeDatasourceUsage(panel);
       }