ipam: netbox: fix adding vms without a name set in the options

ipam: netbox: fix adding vms without a name set in the options

The hostname provided to the methods in the IPAM plugins is empty for
VMs that do not have a name set. We always included the dns_name key
in the request, even if it was undefined, which leads Netbox to return
a 400 Bad Request error. With this patch we omit the dns_name
parameter if it isn't set, so the call succeeds.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
Link: https://lore.proxmox.com/20250408132517.225240-1-s.hanreich@proxmox.com