Custom fields return n/a instead of empty value if field is empty

Custom fields return n/a instead of empty if field is empty. This is a side effect of the check_markup call in civicrm_handler_field_markup.inc (see http://api.drupal.org/api/drupal/modules--filter--filter.module/function/check_markup/6). This patch checks the value returned by check_markup and if it is n/a it returns null.