How to prevent clearing the gpt slot on refresh

I am using the below code sample from GTP docs to refresh the ad automatically in a specific time interval. But, the slot is cleared when it fetches a new ad and the response is empty. Is it possible to prevent clearing the slot until I get a successful response?

googletag.cmd.push(function() {
  var REFRESH_KEY = 'refresh';
  var REFRESH_VALUE = 'true';

  googletag.defineSlot('/6355419/Travel',[728, 90], 'div-for-slot')
      .setTargeting(REFRESH_KEY, REFRESH_VALUE)
      .setTargeting('test', 'event')
      .addService(googletag.pubads());

  // Number of seconds to wait after the slot becomes viewable.
  var SECONDS_TO_WAIT_AFTER_VIEWABILITY = 60;

  googletag.pubads().addEventListener('impressionViewable', function(event) {
    var slot = event.slot;
    if (slot.getTargeting(REFRESH_KEY).indexOf(REFRESH_VALUE) > -1) {
      setTimeout(function() {
        googletag.pubads().refresh([slot]);
      }, SECONDS_TO_WAIT_AFTER_VIEWABILITY * 1000);
    }
  });

  googletag.enableServices();
});