Fix decompress_list IndexError on empty string input

[Anurag-M1]

Summary

decompress_list("") crashes with IndexError because lststr[0] is accessed with no guard for empty input. This breaks the round-trip invariant decompress_list(compress_list([])) == [].

Context / Motivation

compress_list([]) legitimately returns "" (joining an empty list). However, decompress_list("") immediately crashes at line 387 because lststr[0] is accessed without checking for empty input first. An empty task ID list is a valid real-world state.

Fixes #3017

Changes Made

• Added early return if lststr == "": return [] in decompress_list() before the lststr[0] access
• Uses strict == "" (not not lststr) so None still fails loudly — if None slips through, the real bug should be caught upstream

Testing

Added test/unit/test_compress_decompress_list.py with 7 test cases:

Test	What it verifies
test_compress_empty_list	compress_list([]) == ""
test_decompress_empty_string	decompress_list("") == [] (the actual bug)
test_roundtrip_empty	decompress_list(compress_list([])) == []
test_roundtrip[single]	Round-trip with ["x"]
test_roundtrip[simple]	Round-trip with ["a", "b", "c"]
test_roundtrip[common-prefix]	Items with shared prefix trigger : encoding
test_roundtrip_zlib	Force zlib path via zlibmin=0

All 7 tests pass. Full unit test suite unaffected.

$ python3 -m pytest test/unit/test_compress_decompress_list.py -v
7 passed in 0.17s

AI Disclosure

• I used AI tools in preparing this contribution

AI tools were used to help identify the issue and structure the PR. All code was reviewed and understood before submission.

[greptile-apps]

Greptile Summary

Fixes empty-list round trips by making decompress_list("") return an empty list before indexing the input.

• Adds an explicit empty-string guard while preserving failure behavior for None.
• Adds regression and round-trip coverage for empty, plain, prefix-compressed, and zlib-compressed lists.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
metaflow/util.py	Adds a narrowly scoped empty-string guard that prevents the reported IndexError and restores the compression round-trip invariant.
test/unit/test_compress_decompress_list.py	Adds focused regression tests for empty input and representative existing compression modes.

_{Reviews (3): Last reviewed commit: "Merge branch 'master' into fix/decompres..." | Re-trigger Greptile}

[talsperre]

It makes fixes for two separate issues. Let's just do a targeted fix of #3017 and then I can run the tests/merge.

[Anurag-M1]

Updated as requested! Rebased onto master and narrowed down the PR to only include the targeted fix for #3017 along with unit tests.