!13261 Fix DatasetCache C++ API

From: @lixiachen
Reviewed-by: @nsyca
Signed-off-by:

include/api/dual_abi_helper.h

@@ -32,13 +32,15 @@ inline std::vector<char> StringToChar(const std::string &s) { return std::vector
 inline std::string CharToString(const std::vector<char> &c) { return std::string(c.begin(), c.end()); }
 
 inline std::optional<std::vector<char>> OptionalStringToChar(const std::optional<std::string> &s) {
+  if (s == std::nullopt) return std::nullopt;
   std::optional<std::vector<char>> ret = std::vector<char>(s->begin(), s->end());
-  return (s == std::nullopt) ? std::nullopt : ret;
+  return ret;
 }
 
 inline std::optional<std::string> OptionalCharToString(const std::optional<std::vector<char>> &c) {
+  if (c == std::nullopt) return std::nullopt;
   std::optional<std::string> ret = std::string(c->begin(), c->end());
-  return (c == std::nullopt) ? std::nullopt : ret;
+  return ret;
 }
 
 inline std::pair<std::vector<char>, int32_t> PairStringToChar(const std::pair<std::string, int32_t> &s) {