dhcp6: don't require a hardware address
The systemd DHCPv6 client requires a hardware address only to determine the IAID; NM always overrides the IAID with its own and therefore the hwaddr is not used.
Removing such requirement allows DHCPv6 to run over PPP, which is useful with DHCPv6-PD to get a prefix from the ISP.
To test this, I set up a server with pppoe-server, radvd in stateless mode, and the Wide DHCPv6 server providing an address and a prefix. On the client, NM was able to obtain a prefix using both dhcp=dhclient and dhcp=systemd.