Everywhere in our api, possibly missing properties are nullable.
However, to make things easier for everyone, we just default to an
empty url instead, so that users do not have to null-check it.
This adds one more protocol message __dispose__
to dispose a scope and all child objects.
Now, client side does not need a notion of scope anymore -
it just disposes the whole object subtree upon __dispose__.
Server, on the other hand, marks some objects as scopes
and disposes them manually, also asserting that all parents
are proper scopes.
