DLB: Use nullopt default for ARS ID with NHG create

Summary: Using std::nullopt allows passing the attribute to create attr list. Helps with DNX NHG create where this attr is unsupported

Reviewed By: zechengh09

Differential Revision: D67402443

fbshipit-source-id: c0fa7cfa5cd402a13e8e87b6b053fffe3fe6bf69

fboss/agent/hw/sai/switch/SaiNextHopGroupManager.cpp

@@ -64,7 +64,7 @@ SaiNextHopGroupManager::incRefOrAddNextHopGroup(
 
 #if SAI_API_VERSION >= SAI_VERSION(1, 14, 0)
   std::optional<SaiNextHopGroupTraits::Attributes::ArsObjectId> arsObjectId{
-      SAI_NULL_OBJECT_ID};
+      std::nullopt};
   if (FLAGS_flowletSwitchingEnable &&
       platform_->getAsic()->isSupported(HwAsic::Feature::FLOWLET)) {
     auto arsHandlePtr = managerTable_->arsManager().getArsHandle();