[modern] Mutation required children (replace null node through mutation)

Hi everyone,

I’ve been searching through the documentation for an example when you’re performing a mutation which returns a single previously unset/unused node; which I believe used to be called `REQUIRED_CHILDREN`.

For example, say we have a `currentUser` field, which is a `UserNode`:
```
query {
currentUser: null
}
```

We now perform a mutation, say a login action of some sort, which would cause our `currentUser` node to change based on the mutation payload:
```
query {
currentUser: {
id
name
lastLogin
}
}
```

How would a mutation method look to handle this data change; as it needs to be reflected in the UI?

Thank you in advance.