[dashboard api] manage error when data in dashboard table is not valid json (#29999)

* retrieve dashboard api

* Apply suggestions from code review

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Update pkg/api/dashboard.go

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/api/dashboard_test.go

@@ -1009,7 +1009,9 @@ func TestDashboardAPIEndpoint(t *testing.T) {
 				return nil
 			})
 			bus.AddHandler("test", func(query *models.GetDashboardQuery) error {
-				query.Result = &models.Dashboard{Id: 1, Data: &simplejson.Json{}}
+				dataValue, err := simplejson.NewJson([]byte(`{"id": 1, "editable": true, "style": "dark"}`))
+				require.NoError(t, err)
+				query.Result = &models.Dashboard{Id: 1, Data: dataValue}
 				return nil
 			})