Null 값을 설정하려면 컨트롤러가 있습니다.

function display($scope) {
   $scope.obj ={"selected":null};
   $scope.objects = [{id: 1, value: "Yes"}, {id: 0, value: "No"}]
}

다음은 템플릿입니다 -

<div ng-controller = "display">
   <select ng-model = "obj.selected"
      ng-options = "value.id as value.value for value in objects">
      <option value = "">Unknown</option>
   </select>
<br/>
   {{obj}}
</div>