E2E: Fixed e2e test issue due to change in testdata scenario change logic (#23774)

public/app/plugins/datasource/testdata/query_ctrl.ts

@@ -88,8 +88,6 @@ export class TestDataQueryCtrl extends QueryCtrl {
 
   scenarioChanged() {
     this.scenario = _.find(this.scenarioList, { id: this.target.scenarioId });
-    this.target.stringInput = this.scenario.stringInput;
-    this.showLabels = showLabelsFor.includes(this.target.scenarioId);
 
     if (this.target.scenarioId === 'manual_entry') {
       this.target.points = this.target.points || [];
@@ -121,6 +119,9 @@ export class TestDataQueryCtrl extends QueryCtrl {
       delete this.target.stringInput;
     }
 
+    this.target.stringInput = this.scenario.stringInput ?? undefined;
+    this.showLabels = showLabelsFor.includes(this.target.scenarioId);
+
     this.refresh();
   }