security: using new security interfaces of ndn-cpp, such as SecPolicy

Change-Id: Id22c8e711bee7c3723076eda7a3f96c72ca7707c
diff --git a/src/profile.cpp b/src/profile.cpp
index af805b0..d662a70 100644
--- a/src/profile.cpp
+++ b/src/profile.cpp
@@ -9,7 +9,6 @@
  */
 
 #include "profile.h"
-#include "null-ptrs.h"
 #include "logging.h"
 
 using namespace std;
@@ -93,7 +92,7 @@
   if(m_entries.find(profileType) != m_entries.end())
     return m_entries.at(profileType);
 
-  return CHRONOCHAT_NULL_STR;
+  return string();
 }
 
 void