graph-store: Handle missing update-logs gracefully in scries
graph-store: Handle missing update-logs gracefully in scries

If a subscription was kicked before receiving any graph-updates on it,
then the attempt to construct a valid resubscribe path would fail, as
the scry crashes. Fixes this by handling the missing update-log case
correctly.

urbit/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,urbit/urbit