dhcp6: relax data assert in dhcp6_option_parse_string

dhcp6: relax data assert in dhcp6_option_parse_string

dhcp6_option_parse_string is intended to clear strings with length 0,
for consistency. The data assert is too strict for this purpose, so we
will allow data || data_len == 0, similar to other dhcp6_option_parse*
helpers.

Fixes: fde788601be8 ("dhcp6-client: parse RFC8910 captive portal dhcp6 option")