Firebase Auth后端将使用相应的Facebook ID来标识该用户.您可以从currentUser中的providerData列表中获取Facebook ID.将有一个条目，其uid是Facebook ID.因此，没有空标识符.只有电子邮件为空.

Firebase Auth backend will use the corresponding Facebook ID to identify that user. You can get the Facebook ID from the providerData list in the currentUser. There will be an entry whose uid is the Facebook ID. So there is no null identifier. Only the email is null.

如果您想自己为该用户设置电子邮件，也可以使用updateEmail API.

You can also use updateEmail API if you want to set the email yourself on that user.