[C++][Parquet] Boolean encoding has inconsistent implemention

[mapleFU]

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

For PlainEncoder<BooleanType>:

1. For Encoding without arrow:

  void PutSpaced(const bool* src, int num_values, const uint8_t* valid_bits,
                 int64_t valid_bits_offset) override {
    if (valid_bits != NULLPTR) {
      PARQUET_ASSIGN_OR_THROW(auto buffer, ::arrow::AllocateBuffer(num_values * sizeof(T),
                                                                   this->memory_pool()));
      T* data = reinterpret_cast<T*>(buffer->mutable_data());
      int num_valid_values = ::arrow::util::internal::SpacedCompress<T>(
          src, num_values, valid_bits, valid_bits_offset, data);
      Put(data, num_valid_values);
    } else {
      Put(src, num_values);
    }
  }

If values contains null, this only puts length for valid_bits.

2. For Array

  void Put(const ::arrow::Array& values) override {
    if (values.type_id() != ::arrow::Type::BOOL) {
      throw ParquetException("direct put to boolean from " + values.type()->ToString() +
                             " not supported");
    }
    ...
    sink_.UnsafeAdvance(data.length());
  }

This will always output the length with null. These two implement is inconsistent.

Component(s)

C++, Parquet

[mapleFU]

cc @emkornfield @pitrou @wgtmac is this expected?

[mapleFU]

By the way, actually, sink_.UnsafeAdvance(data.length()); will allocate sum-of-array-length-include-null bytes. But finally, data should be encoded into sum-of-array-length-not-include-null bits.