address feedback: narrow guard to empty string and add regression tests

- Update decompress_list to check specifically for "" as a guard.
- Add test/unit/test_util.py with regression tests for empty input.

Author: Vansh0204

metaflow/util.py

@@ -385,7 +385,7 @@ def compress_list(lst, separator=",", rangedelim=":", zlibmarker="!", zlibmin=50
 def decompress_list(lststr, separator=",", rangedelim=":", zlibmarker="!"):
     # Handle the empty-list round-trip: compress_list([]) == "" so we must
     # return [] without touching lststr[0] (which would raise IndexError).
-    if not lststr:
+    if lststr == "":
         return []
     # Three input modes:
     if lststr[0] == zlibmarker:

test/unit/test_util.py

@@ -0,0 +1,9 @@
+from metaflow.util import compress_list, decompress_list
+
+def test_compress_decompress_empty_list():
+    # round-trip must not raise and must return []
+    assert decompress_list(compress_list([])) == []
+
+def test_decompress_empty_string():
+    # direct empty-string input
+    assert decompress_list("") == []