Fix syncing link with props set to None.

[dnwpark]

close #883

Given a schema:

type Target;
type SourceWithProp {
    target: Target {
        lprop: int64;
    };
};

the following code:

with_none = default.SourceWithProp(target=None)
self.client.sync(with_none)

produced an AssertionError:

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)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^

[msullivan]

Don't change this in this PR, but prop ought to be called ptr