Nested folders: Fix moving folder under root (#65684)

* Nested folders: Fix moving folder under root

* Add store test for not empty parent after update

* Modify folder and document store update implementation

Move folder only if NewParentUID is not nil

* Apply suggestion from code review

pkg/services/folder/folderimpl/folder.go

@@ -543,10 +543,14 @@ func (s *Service) Move(ctx context.Context, cmd *folder.MoveFolderCommand) (*fol
 		}
 	}
 
+	newParentUID := ""
+	if cmd.NewParentUID != "" {
+		newParentUID = cmd.NewParentUID
+	}
 	return s.store.Update(ctx, folder.UpdateFolderCommand{
 		UID:          cmd.UID,
 		OrgID:        cmd.OrgID,
-		NewParentUID: &cmd.NewParentUID,
+		NewParentUID: &newParentUID,
 		SignedInUser: cmd.SignedInUser,
 	})
 }