sd-dhcp6: fix check if serverid is set

sd-dhcp6: fix check if serverid is set

Ever since the initial implementation in 631bbe71298ec892f77f44f94feb612646fe6853,
client_parse_message() was supposed to check that the message contains exactly
one serverid. The check that no more than one is given was implemented
correctly, but the check that at least one is given was not. Simplify the whole
thing by making dhcp6_lease_get_serverid() return an error if the id is not
set, and do not require the arguments to be present if the contents of the id
are not needed.