dns: optionally send delete for optional values

dns: optionally send delete for optional values

pbs only deletes the optional values here when they are sent with the
'delete' parameter, in contrast to pve/pmg that don't have a delete
parameter currently and always use the parameters as source of truth.

So to handle that, optionally set deleteEmpty if set from outside

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