Notifications: Use String "0" for null percent
This also ensures it's not mistreated as "missing argument". Ensures that a job without percentage actually shows as 0% and not 1%.
Had a job with "1 file" (so it would switch from indeterminate mode) but then "0%", it is correctly shown as 0% now.
Didn't notice any ki18n warnings on console.
Edited by Kai Uwe Broulik