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.