Dashboards: Fix restoring dashboard to root folder (#89020)

* Fix restoring dashboard to root folder

* use a root folder representation instead of nil

* change root folder by general folder

---------

Co-authored-by: Ezequiel Victorero <ezequiel.victorero@grafana.com>

pkg/services/folder/folderimpl/folder.go

@@ -209,7 +209,10 @@ func (s *Service) Get(ctx context.Context, q *folder.GetFolderQuery) (*folder.Fo
 	var dashFolder *folder.Folder
 	var err error
 	switch {
-	case q.UID != nil && *q.UID != "":
+	case q.UID != nil:
+		if *q.UID == "" {
+			return &folder.GeneralFolder, nil
+		}
 		dashFolder, err = s.getFolderByUID(ctx, q.OrgID, *q.UID)
 		if err != nil {
 			return nil, err