add 'null' to missing value (#1064)

add 'null' to missing value

include/LightGBM/utils/common.h

@@ -248,7 +248,8 @@ inline static const char* Atof(const char* p, double* out) {
     if (cnt > 0) {
       std::string tmp_str(p, cnt);
       std::transform(tmp_str.begin(), tmp_str.end(), tmp_str.begin(), Common::tolower);
-      if (tmp_str == std::string("na") || tmp_str == std::string("nan")) {
+      if (tmp_str == std::string("na") || tmp_str == std::string("nan") ||
+          tmp_str == std::string("null")) {
         *out = NAN;
       } else if (tmp_str == std::string("inf") || tmp_str == std::string("infinity")) {
         *out = sign * 1e308;