networkd/dhcp6: allow layer3 devices without MAC

networkd/dhcp6: allow layer3 devices without MAC

Devices with multicast but without mac addresses i.e. tun devices
are not getting setuped correctly:

$ ip tuntap add mode tun dev tun0
$ ip addr show tun0
16: tun0: <NO-CARRIER,POINTOPOINT,MULTICAST,NOARP,UP> mtu 1500 qdisc fq_codel state DOWN group default qlen 500
    link/none
$ cat /etc/systemd/network/tun0.network
[Match]
Name = tun0

[Network]
Address=192.168.1.1/32
$ ./systemd-networkd
tun0: DHCP6 CLIENT: Failed to set identifier: Invalid argument
tun0: Failed