Method has_null_value

libapp2sys/prebuilt/include/google/protobuf/struct.pb.h:524–526

optional .google.protobuf.NullValue null_value = 1;

// optional .google.protobuf.NullValue null_value = 1;
inline bool Value::has_null_value() const {
  return kind_case() == kNullValue;
}
inline void Value::set_has_null_value() {
  _oneof_case_[0] = kNullValue;
}