Allow `useFirestoreDoc` `DocumentReference` arguments to be nullable

`react-firebase-hooks` allows [the document reference to `useDocument` to be nullable](https://github.com/CSFrequency/react-firebase-hooks/tree/master/firestore#usedocument) and just returns undefined if it is. This was useful for paths relying on nullable info, like `auth.currentUser.uid`:
```js
const [snapshot, loading, error] = useDocument(auth.currentUser && firestore.doc(`users/${auth.currentUser.uid}`));
```
where if the user were not signed in there wouldn't be an error thrown since `useDocument` would just return `undefined`. It would be nice if a similar feature were added to reactfire, as
```js
const { status, data: firebaseDoc } = useFirestoreDoc(auth.currentUser && doc(firestore, 'users', auth.currentUser.uid));
```
won't work as the type of ref is `DocumentReference`, not `DocumentReference | null`,
```js
const { status, data: firebaseDoc } = useFirestoreDoc(doc(firestore, 'users', auth.currentUser?.uid));
```
will throw an error when the user is not signed in (as the path will become invalid), and
```js
if (auth.currentUser) {
const { status, data: firebaseDoc } = useFirestoreDoc(doc(firestore, 'users', auth.currentUser.uid));
}
```
violates the rules of hooks.
Currently relying on a rather abhorrent workaround to resolve this and it would be ideal if reactfire could support this behavior natively.

Start at the useFirestoreDoc entry point and compare its argument behavior with the nullable useDocument behavior described in the issue. Trace how the TypeScript DocumentReference type is handled and inspect any existing Firestore hook tests. Done means nullable references are accepted without an invalid-path error while the hook remains valid to call unconditionally.

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