Handle Empty child IRs in _decompose

[Matt711]

Description

• Closes [BUG]: IndexError in _decompose when running pdsh query 17 with decimal data #20408

I think what is happening is that during expression decomposition, we sometimes end up with a Cast node whose children are all literals. Those children produce Empty IR, so when _decompose filters them out we end getting an IndexError a few lines down.

This PR makes it so we now reuse the parent’s input IR in that case.

I haven't been able to come up with a smaller repro, but PDS-H Q17, doesn't crash anymore.

$ POLARS_GPU_ENABLE_CUDA_MANAGED_MEMORY=0 python python/cudf_polars/cudf_polars/experimental/benchmarks/pdsh.py --path scale-10 --no-print-results --no-summarize --executor streaming --iterations 1 17
Query 17 - Iteration 0 finished in 2.0463s

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[TomAugspurger]

I haven't been able to come up with a smaller repro, but PDS-H Q17, doesn't crash anymore.

Thanks for looking! I also have been struggling to make a reproducer. I think one of us should try once more to write one and if we're unable to then we can merge this.

[vyasr]

I haven't reviewed this yet based on Tom's last comment, but please let me know if you decide you can't find a MRE and it is ready for a review.