zabbix-agent: Treat unset accounting values as 0

E.g. borg-backup.service has unset values when it is not running, but
zabbix doesn't know what to do with a string when it expects an integer
so fix those values here.
Signed-off-by: Florian Pritz <bluewind@xinu.at>

roles/zabbix-agent/templates/zabbix_agentd.conf

@@ -22,7 +22,7 @@ UserParameter=systemd.unit.is-enabled[*],systemctl is-enabled --quiet '$1' && ec
 UserParameter=systemd.failed_units,/usr/local/bin/systemd-units-failed.py
 
 UserParameter=systemd.accounting_discover,/usr/local/bin/systemd-discover-accounting-units.py
-UserParameter=systemd.unit_data[*],systemctl show $(echo '$1' | sed 's#--AT--#@#g') | perl -MJSON -ne 'BEGIN{my %h}; my ($$k,$$v) = split(/=/,$$_,2); chomp($$h{$$k} = $v); END {print encode_json(\%h)};'
+UserParameter=systemd.unit_data[*],systemctl show $(echo '$1' | sed 's#--AT--#@#g') | perl -MJSON -ne 'BEGIN{my %h}; my ($$k,$$v) = split(/=/,$$_,2); chomp($$h{$$k} = ($v =~ s/\[not set\]/0/r)); END {print encode_json(\%h)};'
 
 UserParameter=borg_backup.data,/usr/local/bin/zabbix-borg-timestamps.pl