fix: keep nulls in nullable int32 and int64 decimal parquet columns

[skymanbp]

The DECIMAL conversion for INT32 and INT64 columns took the raw values with toVector and rebuilt the column without its null bitmap, so nulls read as 0.0 and the column was no longer nullable. It now maps the column with mapColumn, which keeps the bitmap, as the timestamp branch does. The new fixture was written by pyarrow 24.0.0 with store_decimal_as_integer=True.

Before:

int32_dec | int64_dec
----------|----------
 Double   |  Double
----------|----------
1.0       | 0.0
0.0       | 1234.5
2.5       | 0.0
0.75      | -5.0

After:

 int32_dec   |  int64_dec
-------------|-------------
Maybe Double | Maybe Double
-------------|-------------
1.0          | null
null         | 1234.5
2.5          | null
0.75         | -5.0

[mchav]

Great catch.