When a Solid device is removed (e.g. a CD is ejected) the notifier tries
to read the attributes although the Source for the UDI has just vanished.
Fixes several QML error messages, i.e. "TypeError: Cannot read property
'...' of undefined" and "Unable to assign [undefined] to QString".
Apparently these errors also have the effect of items showing outdated
state, i.e. optical media still being shown after ejecting it.