std::function doesn't have empty() method

The correct way to perform the check for it in C++11 is to call
std::function::operator bool.

http://en.cppreference.com/w/cpp/utility/functional/function/operator_bool

Change-Id: I4df8daaa758b4d5b382e340b90c4588a83ee11ce

avmedia/source/vlc/wrapper/EventHandler.cxx

@@ -35,7 +35,7 @@ void EventHandler::run()
     {
         mCallbackQueue.pop( callback );
 
-        if ( callback.empty() )
+        if ( callback )
             return;
 
         callback();

avmedia/source/vlc/wrapper/EventManager.cxx

@@ -66,7 +66,7 @@ EventManager::EventManager( Player& player, EventHandler& eh )
 
 void EventManager::registerSignal( int signal, const Callback& callback )
 {
-    if ( callback.empty() )
+    if ( callback )
         libvlc_event_detach( mManager, signal, Handler, this );
     else
         libvlc_event_attach( mManager, signal, Handler, this );