fix: resolve IndexError in decompress_list for empty input

[Vansh0204]

Fix: resolve IndexError in decompress_list for empty input

Fixes #3017

Problem

The decompress_list function was not guarding against empty string input. Since compress_list([]) legitimately returns an empty string "", calling decompress_list(compress_list([])) would immediately crash with:
IndexError: string index out of range at line 387 (if lststr[0] == zlibmarker:).

Fix

Added an early return guard if not lststr: return [] in decompress_list.

Change

• File: metaflow/util.py
• Added check to return [] if input string is empty.

Verification

Verified with a round-trip test:
decompress_list(compress_list([])) now correctly returns [] instead of raising an IndexError.

[greptile-apps]

Greptile Summary

This PR fixes a crash in decompress_list when called with an empty string — the natural output of compress_list([]) — by adding a targeted early-return guard. It also ships a new test/unit/test_util.py covering all three encoding modes and explicitly documenting the pre-existing compress_list([""]) ambiguity.

• Fix is precise: The guard uses lststr == "" (strict equality) rather than if not lststr, so None still raises a TypeError as expected, rather than silently returning [].
• Known limitation is documented: Both the inline comment in util.py and test_compress_empty_string_element_ambiguity make clear that compress_list([]) and compress_list([""]) both produce "", and that decompress_list("") will always return [] — meaning a single empty-string element is unrecoverably lost in a round-trip.
• Test coverage is solid: The three compression modes (plain CSV, prefix-encoded, zlib) are each exercised with round-trip assertions. A local ZLIB_MARKER constant is used instead of a bare "!" literal, tying it to the known default.

Confidence Score: 5/5

• Safe to merge — the one-line guard is minimal, precise, and fully covered by tests.
• The change is a single, narrowly-scoped guard that addresses a well-defined crash path without altering any other behaviour. All previous review concerns (None-safety, ambiguity documentation, test breadth, fragile assertions) have been addressed in this iteration. No new issues were introduced.
• No files require special attention.

Important Files Changed

Filename	Overview
metaflow/util.py	Added a precise if lststr == "": guard in decompress_list to fix IndexError on empty input. The fix correctly uses strict equality (not not lststr) to avoid silently swallowing None, and includes a comment documenting the known compress_list([""]) ambiguity.
test/unit/test_util.py	New test file covering all three compression modes (plain CSV, prefix-encoded, zlib), the empty-list edge case, and explicitly documenting the compress_list([""]) ↔ "" ambiguity as a known limitation.

_{Last reviewed commit: f399208}