Fix syncing unset property with default setting value to None instead of default.

[dnwpark]

Given a schema:

type Az {
    multi val: int64 {
        default := {-1, -2, -3};
    };
};

The following code was failing its assertion:

with_unset = default.Az()
self.client.sync(with_unset)

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

[msullivan]

Sure but I would love if @1st1 looked too