param_to_job: handle --maxsnap 0 on creation

param_to_job: handle --maxsnap 0 on creation

format_job expects the value to be set, so creating a job with '--maxsnap 0' led
to '--maxsnap' being written without an argument, and thus a (for zsync)
unparsable cron file.

However, issuing sync jobs via CLI with '--maxsnap 0' works with 0 being treated
as unlimited. There is a default value, so no need to worry about definedness,
simply pass along the value.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>