onDocumentUpdate event is typed as having data potentially undefined

It seems weird that an update event could contain nothing, if that's the case an update event did not occur, did it ?

```ts
// firestore.d.ts
export declare function onDocumentUpdated(document: Document, handler: (event: FirestoreEvent | undefined, ParamsOf>) => any | Promise): CloudFunction | undefined, ParamsOf>>;
```

It is annoying because it forces an additional, seemingly unnecessary, check.