It can happen the Solid reports an empty string as serial. Let's just use the
last part of the udi as id for now if that happens, it looks reasonable enough
for this. Some exaples:
udi = '/org/freedesktop/UPower/devices/battery_BAT0'
udi = '/org/freedesktop/UPower/devices/keyboard_hid_0018o04F3o280Ex0001_battery'
udi = '/org/freedesktop/UPower/devices/mouse_hidpp_battery_0'