Infinite request loop in useFirestoreCollection

Hello,

the following code results in an infinite loop when user isn't authenticated aka `user.data` is `undefined`. Is this expected?

```
const user = useUser(undefined, { suspense: true });
const subscriptions = useFirestoreCollection(
firestore
.collection('customers')
.doc(user.data?.uid)
.collection('subscriptions')
```

When something random and non-existent is passed to `doc(myId)`, it throws `Missing or insufficient permissions`, when `null` is passed it throws `Function CollectionReference.doc() cannot be called with an empty path`, however when `undefined` is passed the code goes into an infinite loop and triggers hundreds of requests per second.

Reproduce the issue with the shown useUser and useFirestoreCollection snippet, then trace how useFirestoreCollection handles an undefined document path. Compare its behavior with null and a nonexistent id; done means the undefined case no longer causes repeated requests and has a defined outcome.

Scritto dal modello di indicizzazione a partire dal testo della issue.