Cannot set tenantId back to null after setting it to something else

### [REQUIRED] Step 2: Describe your environment

* Android Studio version: VSCode (Flutter development)
* Firebase Component: Auth
* Component version: Whatever [flutterfire](https://github.com/FirebaseExtended/flutterfire) is using

### [REQUIRED] Step 3: Describe the problem

#### Steps to reproduce:

Set tenantId to something. Attempt to set it back to null.

#### Relevant Code:

```
auth.setTenantId(null);
```

#### Details
flutterfire issue: https://github.com/FirebaseExtended/flutterfire/issues/7896#issue-1107115648