scripts: only log session item Id when unhandling it

This MR fixes the following log when unhandling a session item:

m-lua-scripting rescan.lua:73:unhandleLinkable: <WpSiAudioAdapter:0x63386ca7dfe0> unhandling item: nil (nil)

Since the event dispatcher always handles session item events after all the nodes have been removed, we cannot print the associated node name when unhandling the item, so we instead just print the session item Id. This change also logs the session item Id when handling it, so that it is easy to know if the unhandled session item was previously handled or not.