fix(variant): preserve nulls for missing shredded list indexes

[emecii]

Which issue does this PR close?

• Closes [Variant] variant_get returns Variant null instead of SQL NULL for out-of-bounds access on shredded lists #11050.

Rationale for this change

An out-of-bounds list lookup and a present Variant null both produce null child columns after take. Without preserving index validity separately, shredded input turns missing list positions into valid Variant nulls, unlike unshredded input.

What changes are included in this PR?

• Carry the list index validity mask through shredded path traversal.
• Apply that mask to the output while preserving explicit Variant null elements as valid values.
• Update list-like coverage and add a regression test distinguishing an empty list, an explicit null element, and a typed element.

Are these changes tested?

Yes.

• cargo test -p parquet-variant-compute
• cargo clippy -p parquet-variant-compute --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

Are there any user-facing changes?

Yes. variant_get now returns an Arrow null for an out-of-bounds lookup on shredded list-like values, matching unshredded input semantics.

AI usage

OpenAI Codex assisted with investigating the issue, generating the implementation and tests, running the listed local validation, and drafting this PR description.

[sdf-jkl]

I'll take a look tomorrow