[ty] Add tests for empty list/tuple unpacking (#18451)

## Summary

This PR is to address this comment:
https://github.com/astral-sh/ruff/pull/18438#issuecomment-2935344415

## Test Plan

Run mdtest

crates/ty_python_semantic/resources/mdtest/unpacking.md

@@ -851,3 +851,14 @@ def _(arg: tuple[tuple[int, str], Iterable]):
     # revealed: tuple[int | bytes, str | bytes]
     [reveal_type((a, b)) for a, b in arg]
 ```
+
+## Empty
+
+Unpacking an empty tuple or list shouldn't raise any diagnostics.
+
+```py
+[] = []
+() = ()
+[] = ()
+() = []
+```