How to read hyperlink field which has no value with JSOM

When reading a hyperlink field which has no value, I got error from Console:

Unable to get property 'get_url' of undefined or null reference

function onQuerySucceeded(sender, args) {

var listItemInfo = '';

var listItemEnumerator = collListItem.getEnumerator();

while (listItemEnumerator.moveNext()) {
    var oListItem = listItemEnumerator.get_current();
        listItemInfo += '\nID: ' + oListItem.get_id() + 
        '\nTitle: ' + oListItem.get_item('Title') + 
        '\nSummary: ' + oListItem.get_item('Summary') +
        '\nImage url: '+  oListItem.get_item('Image').get_url();
}

alert(listItemInfo.toString());
}