[5.1.x] Fixed #35665 -- Fixed a crash when passing an empty order_by to Window.

This also caused un-ordered sliced prefetches to crash as they rely on Window.

Regression in e16d0c176e that made OrderByList
piggy-back ExpressionList without porting the empty handling that the latter
provided.

Supporting explicit empty ordering on Window functions and slicing is arguably
a foot-gun design due to how backends will return undeterministic results but
this is a problem that requires a larger discussion.

Refs #35064.

Thanks Andrew Backer for the report and Mariusz for the review.

Backport of 602fe961e6 from main.

tests/expressions_window/tests.py

@@ -928,6 +928,20 @@ class WindowFunctionTests(TestCase):
             ),
         )
 
+    def test_empty_ordering(self):
+        """
+        Explicit empty ordering makes little sense but it is something that
+        was historically allowed.
+        """
+        qs = Employee.objects.annotate(
+            sum=Window(
+                expression=Sum("salary"),
+                partition_by="department",
+                order_by=[],
+            )
+        ).order_by("department", "sum")
+        self.assertEqual(len(qs), 12)
+
     def test_related_ordering_with_count(self):
         qs = Employee.objects.annotate(
             department_sum=Window(