applets/batterymonitor: Use "video-display-brightness" for !hasBatteries

f9e39815 ("applets/batterymonitor: Improve for desktop systems") changed the
plasmoid to use a plain icon instead of the CompactRepresentation if there are
no batteries.
While 471cc374 ("applets/batterymonitor: fix CompactRepresentation") brought
the use of the CompactRepresentation back, the plain icon is still used.

For the case that there are no batteries and brightness controls available,
it fell back to "battery", which has no symbolic variant. Change that to use
the symbolic "video-display-brightness" as well.

BUG: 448520


(cherry picked from commit 1b9826d3)