Test in jquery result if json contains only {}?

Question

I have a json from an wordpress endpoint and want to test if testState contains not key and values but only {}.

function testFunction() {
return json_decode('{
    "testData": [
      {
        "_id": "1",
        "name": "testName1"
      },
      {
        "_id": "2",
        "name": "testName2"
      },
    ],
    "testState": {
     }
   }');  }

here the ajax code which is not working

               jQuery.ajax({
                    type: 'POST',
                    dataType: 'json',
                    url: '/wp-admin/admin-ajax.php',
                    data: {
                        action: 'testFunction'
                    },
                    success: function(result) {
                        if(result.serviceState == '{}'){
                            console.log(result);
}

what is the right syntax to do something like if(result.serviceState == '{}')?