/**
  * Load the field value from an input array based on it's name.
  * If not enabled, set to empty string for easier data querying.
  *
  * @param array $input (optional) Array of field names and values. Defaults to $_POST
  **/
 public function set_value_from_input($input = null)
 {
     parent::set_value_from_input($input);
     if ($this->get_value() === null) {
         $this->set_value('');
     }
 }