Prefer = default for default ctors and dtors

Change-Id: I6ef86691902be8d4dfea14b029e4aa59be982854
diff --git a/src/ast_path.hpp b/src/ast_path.hpp
index e097a0c..4e21167 100644
--- a/src/ast_path.hpp
+++ b/src/ast_path.hpp
@@ -63,7 +63,7 @@
 };
 
 struct listElement_ {
-    listElement_() {}
+    listElement_() = default;
     listElement_(const std::string& listName, const ListInstance& keys);
 
     bool operator==(const listElement_& b) const;
@@ -73,7 +73,7 @@
 };
 
 struct list_ {
-    list_() {}
+    list_() = default;
     list_(const std::string& listName);
 
     bool operator==(const list_& b) const;