message: Transform NULL into the empty string
diff: @@ -31,6 +31,9 @@ static bool ParseBoolean(const Value &value, const string &loption) {
  }
  static string ParseString(const Value &value, const string &loption) {
  + if (value.IsNull()) {
  + return string();
  + }
  if (value.type().id() == LogicalTypeId::LIST) {
  auto &children = ListValue::GetChildren(value);
  if (children.size() != 1) {
  