vzdump: pbs: suppress output from upload-log command

vzdump: pbs: suppress output from upload-log command

which ended up in the backup task log and can be confusing to users:

> INFO: Backup finished at 2021-10-11 08:40:38
> Result: {
>       "data": null
> }
> INFO: Backup job finished successfully

For proxmox-backup-server < 2.0.11-1, there was no output for the
upload-log command.

Reported in the community forum:
https://forum.proxmox.com/threads/backup-finishes-ok-but-data-null-info-on-finish.97740/

(cherry picked from commit 76897db1466870b03ed2f2d06db5164fee0b305f)
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>