To set null values, here is the controller −
function display($scope) {
$scope.obj ={"selected":null};
$scope.objects = [{id: 1, value: "Yes"}, {id: 0, value: "No"}]
}The following is the template −
<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>