I have not used the rest api with acf, but generally, in templates, I do something like
$value = get_field('field-name');
if (!$value) {
$value = 'default value';
}
Since I’m assuming that you’re building code in javascript to use the rest api, you’ll need to alter this to check whatever field your looking for in what the api returns.
Like I said, the people that put together the acf to rest plugin will likely be able to answer you’re questions better than anyone.
