mgmt: add FaceQueryFilter::empty()

This commit also simplifies FaceQueryFilter with ndn::optional.

refs #3864

Change-Id: I7cd52101b83ca88df499ca4b507ce93ef6269fd4
diff --git a/tests/unit-tests/mgmt/nfd/face-query-filter.t.cpp b/tests/unit-tests/mgmt/nfd/face-query-filter.t.cpp
index 7e91b8d..d8dbc67 100644
--- a/tests/unit-tests/mgmt/nfd/face-query-filter.t.cpp
+++ b/tests/unit-tests/mgmt/nfd/face-query-filter.t.cpp
@@ -84,6 +84,7 @@
 BOOST_AUTO_TEST_CASE(Equality)
 {
   FaceQueryFilter filter1, filter2;
+  BOOST_CHECK_EQUAL(filter1.empty(), true);
   BOOST_CHECK_EQUAL(filter1, filter2);
 
   filter1.setFaceId(100)
@@ -93,6 +94,7 @@
          .setFaceScope(FACE_SCOPE_LOCAL)
          .setFacePersistency(FACE_PERSISTENCY_ON_DEMAND)
          .setLinkType(LINK_TYPE_MULTI_ACCESS);
+  BOOST_CHECK_EQUAL(filter1.empty(), false);
   BOOST_CHECK_NE(filter1, filter2);
 
   filter2 = filter1;