Actually, ACF returns NULL (which is different than an empty string) if the field is not defined for a post, meaning there is no meta value in the database associated with the post.
You can get a list of all of the field groups attached to a specific post and then get all the fields in those groups.
// get field groups for the post
// returns an array of field groups
$groups = acf_get_field_groups(array('post_id' => $post_id));
// get fields in a group
// index into the array returned above
$fields = acf_get_fields($groups[$index]['key']);
