[next] don't link filter stream nodes if its target.object node is not found

Currently, if a filter stream node has a defined target using the target.object property, and the defined target node does not exist, it fallbacks to the default device. This can be a problem if we only want to use the filter for a specific device, and not link it at all if the device does not exist.

It would be great if we could add an option in wireplumber's next branch so that the policy can do that. I think the node.dont-reconnect property is expected to do this, but it seems to be broken on next.

Also, I think it might be better to automatically do this by default for all filter nodes whose defined target does not exist. It makes sense to fallback to the default device if the stream node is a client, but it does not really make a lot of sense to do that for filters nodes with a defined target, right?

See #501 (closed),