admin/auth/updateUser for MFA not working

Hello. Discovered the following issue using firebase admin v10. I can successfully enroll a user for the MFA process by calling admin.auth().updateUser(...) with multiFactor/enrolledFactors provided. But removing enrolled factors is not successful (https://firebase.google.com/docs/auth/admin/manage-mfa-users):

```
admin.auth().updateUser({
uid: '123456789',
multiFactor: {
enrolledFactors: null,
},
})
.then((userRecord) => {
console.log(userRecord.multiFactor);
})
.catch((error) => {
console.log(error);
});
```
The factors remain in DB. I can see the same discussed for Golang as well here https://github.com/firebase/firebase-admin-go/pull/530/files.

Start by reproducing the admin.auth().updateUser(...) call shown in the issue with multiFactor.enrolledFactors set to null, and compare its behavior with the MFA management documentation and linked Go pull request. Done means removing enrolled factors through this call no longer leaves them in the database and the returned user record reflects the removal.

索引モデルが issue の本文から書いたものです。