AngularJs: Set Model null on change in Directive

I Want to change the model in directive , on change of checkbox in directive .

   <input type="checkbox" ng-model="isCheck" />
        <select ng-model="empNmae" ng-setempty="isCheck" 
        ng-options="em.Value as em.Name for em in emp">
          <option value="">Select
        </select>
        {{empNmae}}

JS

// Code goes here
var app=angular.module('myApp',[]);
app.controller("mycntrl", function ($scope) {
$scope.emp=[{
  "Name":"Amit",
  "Value":101
},{
  "Name":"Amit2",
  "Value":103
},{
  "Name":"Amit3",
  "Value":102
}];
$scope.empNmae=102;

});

app.directive('ngSetempty', function () {
    return {
        restrict: 'A',
        require: 'ngModel',
        link: function (scope, element, attrs, contro) {
            scope.$watch(attrs.ngSetempty,function(scope, element, attrsArg, contro){
            if(this.last){
              alert(attrsArg.empNmae);
             // Set Model of select Null
            }else{
              //Reset the model of select to intial 
            }
            });
        }
    };
});

I want to set model of select to null , if checkbox is checked, and he uncheck the checkbox, then set the model of select to intial value.

MY Plunker