fix(encoding): handle empty rows in variable packed struct decode

[Xuanwo]

In full-zip variable packed decoding, rep/def may produce visible rows with empty payloads (for null/invalid items). The decoder previously assumed every visible row had bytes for each child and failed with Packed struct fixed child exceeds row bounds.

This happened during write new tables with blob v2.

Summary

• fix PackedStructVariablePerValueDecompressor to handle empty packed rows (row_start == row_end)
• append one per-child placeholder value for empty rows so child builders remain row-aligned

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Parts of this PR were drafted with assistance from Codex (with gpt-5.3-codex) and fully reviewed and edited by me. I take full responsibility for all changes.

[github-actions]

PR Review: fix(encoding): handle empty rows in variable packed struct decode

Summary: This is a well-targeted bug fix for handling empty rows in variable packed struct decoding.

Assessment: ✅ LGTM

No P0/P1 issues identified.

The fix correctly handles the edge case where rep/def produces visible rows with empty payloads (row_start == row_end). The approach of pre-creating empty placeholder values and appending them when empty rows are encountered is sound and maintains child row alignment as expected.

Minor observations (non-blocking):

• The test coverage is adequate for the specific bug being fixed
• Pre-allocating empty_value for each field introduces minimal overhead and is only used when needed

The implementation follows the existing code patterns well.

[codecov]

Codecov Report

❌ Patch coverage is 92.63158% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...st/lance-encoding/src/encodings/physical/packed.rs	92.63%	6 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[westonpace]

Nice catch!