don't return empty tags strings to lua

src/extractor/scripting_environment_lua.cpp

@@ -34,7 +34,7 @@ namespace
 template <class T>
 auto get_value_by_key(T const &object, const char *key) -> decltype(object.get_value_by_key(key))
 {
-    return object.get_value_by_key(key, "");
+    return object.get_value_by_key(key);
 }
 
 template <class T> double latToDouble(T const &object)