Only write DHCP6 gw when given a value. Fixes #11454
(cherry picked from commit 78ca7d58c6cc706b5c6aeb8c00c6b4e2b5c841cd)
Only write DHCP6 gw when given a value. Fixes #11454
(cherry picked from commit 78ca7d58c6cc706b5c6aeb8c00c6b4e2b5c841cd)