don't show 0% when no battery

Summary:
if there is no battery don't show 0%, but a - on
the badge. still show the badge to show the option is not broken,
but don't show a weird 0% of a non existing battery

Test Plan: works on both systems with or without battery

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: broulik, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4606