fix disappeared status of folder after restart syncthing

sometimes after restart process syncthing '/rest/db/status' for folder may return data with 'state' = empty string

gui/scripts/syncthing/core/controllers/syncthingController.js

@@ -522,6 +522,11 @@ angular.module('syncthing.core')
                 return 'stopped'; // legacy, the state is called "stopped" in the GUI
             }
             
+            // after restart syncthing process state may be empty
+            if (!$scope.model[folderCfg.id].state) {
+                return 'unknown'; 
+            }
+
             return '' + $scope.model[folderCfg.id].state;
         };