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(); });