fix: Preserve sort with empty group {}

[max-sixty]

Summary

• Fix sorting being incorrectly dropped when using group {} (sort a | take 1)
• Empty group {} should preserve inner sort since there's no actual partitioning

Before:

SELECT * FROM foo LIMIT 1

After:

SELECT * FROM foo ORDER BY a LIMIT 1

Test plan

• Added test test_group_empty_preserves_sort
• Verified non-empty groups still work correctly (sort stays internal to partition)
• All 631 tests pass
• Lints pass

Fixes #5100

🤖 Generated with Claude Code