Check that page exists before linking it

[AJenbo]

Probably also limit the input to int

https://sentry.io/anders-jenbo/agcms/issues/416937300/

TypeError: Argument 3 passed to AGCMS\Entity\Table::updateRow() must be of the type integer or null, string given, called in /customers/7/8/5/huntershouse.dk/httpd.www/inc/Controller/Admin/TableController.php on line 104
  File "inc/Entity/Table.php", line 315, in updateRow
    public function updateRow(int $rowId, array $cells, int $link = null): void
  File "inc/Controller/Admin/TableController.php", line 104, in updateRow
    $table->updateRow($rowId, $cells, $link);
  File "inc/Controller/Admin/TableController.php", line 0, in call_user_func_array
    AGCMS\Controller\Admin\TableController->updateRow
  File "inc/Application.php", line 240, in AGCMS\{closure}
    return call_user_func_array([new $route['controller'](), $route['action']], $matches);
  File "inc/Middleware/Utf8Url.php", line 25, in handle
    return $next($request);
...
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