firebase.auth.currentUser.linkWith functions have confusing return type.

### [REQUIRED] Describe your environment

* Operating System version: N/A
* Browser version: N/A
* Firebase SDK version: 6.4.2
* Firebase Product: auth

### [REQUIRED] Describe the problem
Successful calls to the class of functions with the signature `firebase.auth.currentUser.linkWith*` if successful return an object of the type `firebase.auth.UserCredential`. Which has a `user` property of type `firebase.User | null`. It should not be the case if this call is correct that the `user` property is null. This leads to unnecessary developer confusion as well as fighting with the TypeScript type engine.

**Suggested change is a non-nullable version of `firebase.auth.UserCredential` something of the effect:**

`type NonNullUserCredential = firebase.auth.UserCredential & {user: firebase.User}
`

#### Steps to reproduce:

N/A
#### Relevant Code:
N/A

https://stackblitz.com/fork/firebase-issue-sandbox

```javascript
// TODO(you): code here to reproduce the problem
```