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GH-34326: [C++][Parquet] Page null_count is incorrect if stats is disabled (#34327)

### Rationale for this change

Parquet ColumnWriter obtains null_count of a page from page stats as below ([link](https://github.com/apache/arrow/blob/main/cpp/src/parquet/column_writer.cc#L952))
```cpp
  EncodedStatistics page_stats = GetPageStatistics();

  int32_t null_count = static_cast<int32_t>(page_stats.null_count);

  DataPageV2 page(combined, num_values, null_count, num_rows, encoding_,
                    def_levels_byte_length, rep_levels_byte_length, uncompressed_size,
                    pager_->has_compressor(), page_stats);
```

However, the null_count is uninitialized if page stat is not enabled:
```cpp
  EncodedStatistics GetPageStatistics() override {
    EncodedStatistics result;
    if (page_statistics_) result = page_statistics_->Encode();
    return result;
  }
```

### What changes are included in this PR?

ColumnWriter collects null_count by itself. To be safe, it also checks that from page stats if available.

### Are these changes tested?

Added a test case to cover null counts of optional and repeated fields are properly set.

### Are there any user-facing changes?

No.
* Closes: #34326

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>