[C++][Parquet] Statistics Merge ignore setting has flag

[mapleFU]

Describe the bug, including details regarding any error messages, version, and platform.

In src/parquet/statistics.cc:

   void Merge(const TypedStatistics<DType>& other) override {
     this->num_values_ += other.num_values();
     if (other.HasNullCount()) {
-      this->statistics_.null_count += other.null_count();
+      this->IncrementNullCount(other.null_count());
     }
     if (other.HasDistinctCount()) {
-      this->statistics_.distinct_count += other.distinct_count();
+      this->IncrementDistinctCount(other.distinct_count());
     }
     if (other.HasMinMax()) {

The original code ignore setting has flag.

Component(s)

C++, Parquet

[mapleFU]

Probly the syntax here is trickey. Our Statistics will only have has_null_count_ to be true. I will have a survey on parquet-mr implementions.