set default value to empty array

in case if items source is not defined in $scope when initializing multiselect instance of ui-select, we'll get an undefined from ctrl.parserResult.source($scope). then next line will throw an exception when truing to filter undefined.

Author: ivanvoznyakovsky

src/select.js

@@ -201,7 +201,7 @@ uis.controller('uiSelectCtrl',
     }
 
     function setPlainItems(items) {
-      ctrl.items = items;
+      ctrl.items = items || [];
     }
 
     ctrl.setItemsFn = groupByExp ? updateGroups : setPlainItems;