Assigning to a required link on an update does not work when coalescing with the existing link

Schema:

```esdl
type User {}

type Session {
required expires: datetime;
required user: User;
}
```

Given the following query:

```edgeql
with
user_id := $user_id,
user := (select User filter .id = user_id),
expires := $expires,
session := $session_id,
update session set {
expires := expires ?? .expires,
user := user ?? .user,
};
```

This gives an error:

```
MissingRequiredError: missing value for required link 'user' of object type 'default::Session'v
```

But this does not give an error:

```edgeql
user := assert_exists(user ?? .user),
```

---

Notice that `expires` (which is a required _property_) does not seem to have an issue inferring that the coalesce expression is `AT_LEAST_ONE`, but for the required `user` _link_, it requires wrapping with `assert_exists`.