Fix baloorunner systemd unit showing up as failed when indexing disabled

baloorunner immediately exits with 255 when file indexing is disabled in configuration. (https://invent.kde.org/plasma/plasma-workspace/-/blob/master/runners/baloo/baloosearchrunner.cpp#L48)

This made the systemd unit have status: failed

This patch adds a condition so that the unit is not run at all if not needed