Fix syncing unset property with default setting value to None instead of default. (#910)

Given a schema:
```edgeql
type Az {
    multi val: int64 {
        default := {-1, -2, -3};
    };
};
```

The following code was failing its assertion:
```py
with_unset = default.Az()
self.client.sync(with_unset)

assert with_unset.val == [-1, -2, -3]
```