monitors/alsa: handle node activation failure
When node activation fails, it won't be created and its object-removed signal can be called with some properties missing. This breaks node name deduplication, causing error
wplua: [string "alsa.lua"]:182: attempt to concatenate a nil value (local 'node_name')
It occurs e.g. when switching ALSA device profile, while some application has opened the device with ALSA and is keeping it busy.
Fix by handling the activation failure, and tolerating missing property in object-removed.