Incorrect missing field name - seems to pick name of first null field, even if field is optional

Consider this type:

```f#
type A = { A: int option; B: bool }
```

When deserializing this:

```f#
"{\"A\": null}"
```

It fails with this message:

> Missing field for record type A: A

It should fail with this message:

> Missing field for record type A: B