fix: keep nulls in nullable int32 and int64 decimal parquet columns - #233

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, haskell-actions/run-fourmolu@v11. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/