Unable to set userId to null

**Describe the bug**
setUserId method does not accept null.

https://github.com/capacitor-community/firebase-analytics/blob/master/src/web.ts#L83

**To Reproduce**
```
FirebaseAnalytics.setUserId({
userId: null,
});
```

**Expected behavior**
When the user logs out, it must be possible to assign null value to it.

See here: https://firebase.google.com/docs/reference/android/com/google/firebase/analytics/FirebaseAnalytics#public-void-setuserid-string-id

Inspect src/web.ts around line 83 and reproduce the setUserId call with userId: null. Compare the expected logout behavior with the linked Firebase Analytics reference; done means the call accepts null and clears the user ID without an error.

Written by the indexing model from the issue text.