Fix a few issues found during fine-tuning (#2582)

ming030890

2025-07-04 a750595594321833b48dc19798eed66876a100b4

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#include "yaml-cpp/null.h"
 
namespace YAML {
_Null Null;
 
bool IsNullString(const std::string& str) {
  return str.empty() || str == "~" || str == "null" || str == "Null" ||
         str == "NULL";
}
}