Support nullable @Reference.

[sgr-ksmt]

FireSnapshot does not currently support nullable @Reference.
So, the code that like below will be failed to decode.

struct Model: SnapshotData {
    var name: String
    @Reference<User> var user = nil
}

{
  "name": "Mike",
}

(user: undefined)

---

But, another code that like below will be succeeded.

struct Model: SnapshotData {
    var name: String
    @Reference<User> var user = nil
}

{
  "name": "Mike",
  "user": null
}

---

ToDo

• Implement NullableReference that is allowed {key: undefined}.

[SebastianBO]

Is this done? Moving away from Ballcap-Firestore to this - are you motivated to keep on going? :)