Add default values for the database columns

src/database/column.hpp

@@ -5,8 +5,12 @@
 template <typename T>
 struct Column
 {
+    Column(T default_value):
+        value{default_value} {}
+    Column():
+        value{} {}
     using real_type = T;
-    T value;
+    T value{};
 };
 
 struct Id: Column<std::size_t> { static constexpr auto name = "id_";