Internal DHCPv4 client requires a valid MAC address for functioning.
Just always require a MAC address to start DHCP, both v4 and v6.

We have no MAC address for example on Layer3 devices like tun or wireguard.

Also, before "0a797bdc systemd/dhcp: fix assertion starting DHCP
client without MAC address", if we tired to start sd_dhcp_client without
setting a MAC address, an assertion was triggered.

(cherry picked from commit e8fa75ce)