pkg/node: Set empty string if address is nil
pkg/node: Set empty string if address is nil

net.IP.String() return "<nil>" on empty which gets printed
as
          "ipv6": {
            "ip": "\u003cnil\u003e"
          }
in cilium status -o json

Signed-off-by: Nirmoy Das <6ad020fa725fa73abb9a553bfe35b16e2f80a6a0@suse.de>

cilium/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,cilium-team/cilium,michi-covalent/cilium,cilium/cilium,cilium-team/cilium,michi-covalent/cilium