`generateVerifyAndChangeEmailLink` only works for accounts with existing email

* Operating System version: Firebase Cloud
* Firebase SDK version: 12.6.0
* Firebase Product: auth
* Node.js version: 20
* NPM version: 10

#### Steps to reproduce:

In our app, we allow social login with Facebook (and more, but irrelevant). When authenticating with Facebook, the user can decide to not provide his email address. Since our whole application builds arounds mails, we ask them to provide one.

We activated email enumeration protection recently!

Since we can no longer set that mail in the frontend code and we want styled/custom mails, we would like to use `generateVerifyAndChangeEmailLink` in the admin library. But it seems impossible to call that function, if we don't provide an existing email.

This is, how the user looks in Firebase Console

#### Relevant Code:

```
// I tried different things for the existing mail, but all failed
await getAuth().generateVerifyAndChangeEmailLink('-', email, actionCodeSettings);
```

首先跟踪 Firebase Admin Node.js 身份验证实现中的 generateVerifyAndChangeEmailLink 及其现有测试，并使用针对缺少电子邮件的用户的已报告调用作为复现用例。完成标准是：在启用电子邮件枚举保护的情况下，仍能为此类帐户生成链接，并且为已报告的场景提供测试覆盖。

由索引模型根据 Issue 内容生成。