message: Interpret 'null' json values as empty values in optional fields.
  Otherwise, casting a 'null' value to the final object (e.g. a string)
  will result in an exception. This handles it as if it was a non-set
  value.
diff: @@ -290,7 +290,8 @@ void GenerateCode(const std::string &filename,
  std::string access_deref = access_call + ".";
  if (p.is_optional) {
  fprintf(out,
  - "%*sif (auto found = j.find(\"%s\"); found != j.end()) {\n",
  + "%*sif (auto found = j.find(\"%s\"); found != j.end() && "
  + "!found->is_null()) {\n",
  indent, "", p.name.c_str());
  indent += 4;
  fprintf(out, "%*shas_%s = true;\n", indent, "", p.name.c_str());
  