validateTimestampRange Fails on One of valid_from and valid_through

If calling code provides only one of valid_from or valid_through, the logic in validateTimestampRange will through an error because neither isset() will return true, so r{{eturn}}{{ _txt('er.unknown', array($eval_field));}} will execute.

This happens in certain OrgIdentitySource plugins, which might set only one of these fields, depending on what data is returned from the source. In particular, SalesforceSource and NetForumSource trigger this error, but FileSource does not.

We probably need something like

} elseif(empty($this->data[$this->name][$eval_field]) && empty($this->data[$this->alias][$eval_field])) {

  return true;

} else ...