Parquet: Only fill in null values for string lengths and list offsets

[pmattione-nvidia]

Followup to PR# 19643. This changes it so the parquet decode doesn't fill in null values for anything except string lengths and list offsets. This matches the updated dev guide. Benchmarks show no measurable improvement, but it's still a good idea to do less work, as it will help once other bottlenecks are removed.

This also includes minor changes to simplify thread-out-of-bounds checking in the decode loops.

Checklist

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