NetworkPkg/Dhcp6Dxe: Handle the Nil UUID case

NetworkPkg/Dhcp6Dxe: Handle the Nil UUID case

Nil UUID is a special case with all zeros value. This
patch is to handle this case to avoid the invalid DUID.

Cc: Naveen Santhapur <naveens@amiindia.co.in>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>