Change lookupNSSByUUID to return boost::optional<NamespaceString> instead of NamespaceString

Right now, we have lookupUUIDByNSS which returns boost::optional<CollectionUUID> and lookupNSSByUUID which returns NamespaceString. We should change lookupNSSByUUID to return boost::optional<NamespaceString> to avoid any confusion, as we can return an empty NamespaceString object if the NSS is not found by the UUID and to match the two functions behaviour.