fix #3976: api/backup: make schedule/starttime truly optional on update

fix #3976: api/backup: make schedule/starttime truly optional on update

on create we require either starttime (+dow) or a schedule, but when
updating an existing job, this is not necessary

before we changed to schedules, the starttime was not optional either on
update, but i think there is no reason to require the user to send the
schedule/startime along every time.

the gui will send all values every time, so that was never a problem there

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>