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/Dhcp6Dxe.inf

@@ -5,7 +5,7 @@
 #  and other configuration parameters from DHCPv6 servers.
 #
 #  (C) Copyright 2015 Hewlett-Packard Development Company, L.P.<BR>
-#  Copyright (c) 2009 - 2014, 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
@@ -74,6 +74,8 @@
   gEfiDhcp6ProtocolGuid                              ## BY_START
   gEfiIp6ConfigProtocolGuid                          ## TO_START
 
+[Guids]  
+  gZeroGuid                   ## SOMETIMES_CONSUMES   ## GUID
  
 [Pcd]
   gEfiNetworkPkgTokenSpaceGuid.PcdDhcp6UidType       ## SOMETIMES_CONSUMES