Fix syncing link with props set to None. (#901)

Given a schema:
```edgeql
type Target;
type SourceWithProp {
    target: Target {
        lprop: int64;
    };
};
```

the following code:
```py
with_none = default.SourceWithProp(target=None)
self.client.sync(with_none)
```

produced an `AssertionError:`
```cmd
File ".../edgedb-python/gel/_internal/_save.py", line 1105, in make_save_executor_constructor
) = make_plan(
    ^^^^^^^^^^
File ".../edgedb-python/gel/_internal/_save.py", line 737, in make_plan
assert isinstance(val, ProxyModel)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
```