message: fix(node): Ensure that self._handler exists before calling it in LinkedErrors
  Bring back a definedness check for `self._handler` in the global event processor of the LinkedErrors integration for Node.
  The check was removed in but apparently it is still necessary.
diff: @@ -47,7 +47,7 @@ export class LinkedErrors implements Integration {
  const hub = getCurrentHub();
  const self = hub.getIntegration(LinkedErrors);
  const client = hub.getClient<NodeClient>();
  - if (client && self) {
  + if (client && self && self._handler && typeof self._handler === 'function') {
  await self._handler(client.getOptions().stackParser, event, hint);
  }
  return event;
  