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.