policy-node: wait for nodes when we become unlinked

If we were linked before but our node is removed, wait until a new node becomes available instead of failing.

This fixes an issue where there is only 1 sink available and the card profile is toggeled between pro and stereo. After the profile is toggled, the sink is removed and the node would be killed with an error because there is no fallback sink. The fix is similar to the pipewire-media-session logic.