mgmt: Fix issue with readvertisement commands always returning error code

NFD RIB commands in NLSR were previously always returning 406 errors due
to not having correct default return values for optional post-processing.
This does not appear to impede functionality but does cause incorrect
behavior on the NFD side and is blocking other changes.

Also fixes an issue where NLSR was not sending correctly formatted responses
back to commands from NFD which was raising errors.

Refs #5358

Change-Id: Iffe6a2416a6d7eafa44e17c821bf903c12ecb104
diff --git a/tests/update/test-save-delete-prefix.cpp b/tests/update/test-save-delete-prefix.cpp
index 5c582cd..5adec17 100644
--- a/tests/update/test-save-delete-prefix.cpp
+++ b/tests/update/test-save-delete-prefix.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2024,  The University of Memphis,
+ * Copyright (c) 2014-2025,  The University of Memphis,
  *                           Regents of the University of California,
  *                           Arizona Board of Regents.
  *
@@ -212,7 +212,7 @@
   face.receive(advertiseWithdraw("/prefix/to/save", "advertise", true));
   this->advanceClocks(ndn::time::milliseconds(10));
   BOOST_REQUIRE(counter == 1);
-  BOOST_CHECK_EQUAL(getResponseCode(), 406);
+  BOOST_CHECK_EQUAL(getResponseCode(), 500);
   face.sentData.clear();
 
   // only withdraw