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>

NetworkPkg/Dhcp6Dxe/Dhcp6Impl.h

@@ -1,7 +1,7 @@
 /** @file
   Dhcp6 internal data structure and definition declaration.
 
-  Copyright (c) 2009 - 2016, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2009 - 2017, Intel Corporation. All rights reserved.<BR>
 
   This program and the accompanying materials
   are licensed and made available under the terms and conditions of the BSD License
@@ -37,6 +37,7 @@
 #include <Library/BaseLib.h>
 #include <Library/NetLib.h>
 #include <Library/PrintLib.h>
+#include <Guid/ZeroGuid.h>
 
 
 typedef struct _DHCP6_IA_CB    DHCP6_IA_CB;