Dashboard: Fix export for sharing when panels use default data source (#19315)

* PanelModel: moved datasource: null away from defaults that are removed

* Added unit test

public/app/features/dashboard/state/PanelModel.test.ts

@@ -93,6 +93,11 @@ describe('PanelModel', () => {
       expect(saveModel.gridPos).toBe(undefined);
     });
 
+    it('getSaveModel should not remove datasource default', () => {
+      const saveModel = model.getSaveModel();
+      expect(saveModel.datasource).toBe(null);
+    });
+
     it('getSaveModel should remove nonPersistedProperties', () => {
       const saveModel = model.getSaveModel();
       expect(saveModel.events).toBe(undefined);

public/app/features/dashboard/state/PanelModel.ts

@@ -69,7 +69,6 @@ const mustKeepProps: { [str: string]: boolean } = {
 
 const defaults: any = {
   gridPos: { x: 0, y: 0, h: 3, w: 6 },
-  datasource: null,
   targets: [{ refId: 'A' }],
   cachedPluginOptions: {},
   transparent: false,
@@ -126,6 +125,10 @@ export class PanelModel {
   constructor(model: any) {
     this.events = new Emitter();
 
+    // should not be part of defaults as defaults are removed in save model and
+    // this should not be removed in save model as exporter needs to templatize it
+    this.datasource = null;
+
     // copy properties from persisted model
     for (const property in model) {
       (this as any)[property] = model[property];