[applets/batterymonitor] Don't show percentage overlay with no batteries
requested to merge ngraham/plasma-workspace:dont-show-battery-percentage-overlay-when-there-are-no-batteries into master
There's no point, since it always says "-" and just covers up the icon. Let's only show the overlay if there are actually any batteries.
BUG: 435890 FIXED-IN: 5.22
Dunno how to test since my only machines have batteries.