channeldb: within AddrsForNode don't fail if no graph node is found
channeldb: within AddrsForNode don't fail if no graph node is found

In this commit, we modify the `AddrsForNode` method to not fail if no
graph node is found. We do this as when the backup is being
restored/created, it's possible that we don't yet have a
NodeAnnouncement for that node.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd