getRedirectResult() always returns null (AF7+FB9)

### Version info

**Angular: 13.1.2**

**Firebase: 9.6.3**

**AngularFire: 7.2.0 / 7.3.0**

### How to reproduce these conditions

on my ngInit():

```javascript
getRedirectResult().then(result => { console.log(result)})
```

### Debug output

i always get null.

i tried using the /compat/auth, i get

```javascript
{result : { credentials: null, user: null }}
```
### Expected behavior

results should contain credentials and user.

### Actual behavior

no results at all.
Works properly in AF 6.1.4 + FB 8.2.1. Encountered after upgrade.

onAuthStageChanged() works; it detects the user correctly. Problem is, There is a subscription to auth,state already running on a different module (which i am prohibited to modify). Subscribing to onAuthStageChanged() creates some sort of pseudo-deadlock (sometimes it resolves, sometimes it doesnt).

My temp workaround is to use signInWithPopup() but that disables mobile compatibility.
Tips on overcoming this are highly appreciated. Thank you

Start with the getRedirectResult() call shown in the reproduction and compare its behavior across AngularFire 7.2.0/7.3.0 with Firebase 9.6.3 versus AngularFire 6.1.4 with Firebase 8.2.1. Check the /compat/auth result and the interaction with the existing auth state subscription; done means redirect sign-in returns credentials and a user without relying on signInWithPopup().

由索引模型根據 Issue 內容生成。