ipam: netbox: no conditional assignments for descriptions

ipam: netbox: no conditional assignments for descriptions

While it should make practically no difference, it opens up potential
errors in the future, so just remove the conditional assignments and
explicitly define the variable as undef, so the intention is more
clear.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
Tested-by: Hannes Duerr <h.duerr@proxmox.com>
Link: https://lore.proxmox.com/20250310085103.30549-4-s.hanreich@proxmox.com
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>