media-session: if no node found on reconnect, unlink but no error

If a reconnecting node was linked at least once successfully, and a
re-link ends up not finding any node to link to, don't raise an error.
Instead, just unlink the node, and wait for a suitable sink to appear
later.

This fixes the issue that some Pulseaudio apps have to be restarted
if all devices go away, before they start working.

It also seems like the right thing to do, because if you remove links
manually, this does not cause errors to occur.