Fix D65159011 regression

Summary:
D65159011 added an empty folly::IPAddress: https://fburl.com/code/qgcdf74t
This fails on in toSaiIpAddress as its folly::address_unreachable.

Fix it by returning some (all 0) IPv6 address. This is fine as since this IP is not used anywhere.

Reviewed By: maxwindiff

Differential Revision: D65824443

fbshipit-source-id: d8e1a662a49626c9d5df2484bb0e8043d60588c0

fboss/agent/hw/sai/api/SaiDefaultAttributeValues.h

@@ -196,7 +196,7 @@ struct SaiHostifUserDefinedTrapDefaultPriority {
 
 struct SaiIpAddressDefault {
   folly::IPAddress operator()() const {
-    return folly::IPAddress{};
+    return folly::IPAddressV6("::");
   }
 };