don't compare std::function to nullptr ref microsoft#4633

src/io/dataset_loader.cpp

@@ -1143,7 +1143,7 @@ void DatasetLoader::ExtractFeaturesFromMemory(std::vector<std::string>* text_dat
  double tmp_label = 0.0f;
  auto& ref_text_data = *text_data;
  std::vector<float> feature_row(dataset->num_features_);
- if (predict_fun_ == nullptr) {
+ if (!predict_fun_) {
  OMP_INIT_EX();
  // if doesn't need to prediction with initial model
  #pragma omp parallel for schedule(static) private(oneline_features) firstprivate(tmp_label, feature_row)
@@ -1262,7 +1262,7 @@ void DatasetLoader::ExtractFeaturesFromMemory(std::vector<std::string>* text_dat
 void DatasetLoader::ExtractFeaturesFromFile(const char* filename, const Parser* parser,
  const std::vector<data_size_t>& used_data_indices, Dataset* dataset) {
  std::vector<double> init_score;
- if (predict_fun_ != nullptr) {
+ if (predict_fun_) {
  init_score = std::vector<double>(dataset->num_data_ * num_class_);
  }
  std::function<void(data_size_t, const std::vector<std::string>&)> process_fun =