Fix "Unable to assign [undefined] to int" log

The sddm breeze theme battery icon is trying to access the battery
percent. In a batteryless system it produces the syslog message:
Nov 14 22:28:23 samd sddm-greeter[3210]: file:///usr/share/sddm/themes/breeze/components/Battery.qml:39:18: Unable to assign [undefined] to int

Based on https://gist.github.com/Zren/4e5709d842965227088f6e1d3fd42016

Debian-Bug: https://bugs.debian.org/844194