enable_window_topn panics on WHERE rn < 1 (PartitionedTopK requires k > 0)

[viirya]

Describe the bug

With datafusion.optimizer.enable_window_topn = true, a query whose filter is rn < 1 (or the flipped 1 > rn) over a partitioned ROW_NUMBER()/RANK() panics:

thread '...' panicked at datafusion/physical-plan/src/topk/mod.rs:1294:
PartitionedTopK requires k > 0

ROW_NUMBER()/RANK() are always >= 1, so rn < 1 matches no rows and the correct result is an empty relation — it should not panic. With the optimization disabled the same query correctly returns no rows.

To Reproduce

SET datafusion.optimizer.enable_window_topn = true;

SELECT * FROM (
  SELECT *, ROW_NUMBER() OVER (PARTITION BY pk ORDER BY val) AS rn
  FROM t
) WHERE rn < 1;          -- panics; also `WHERE 1 > rn`

Root cause

The WindowTopN rule's extract_window_limit maps rn < K to a fetch of K - 1 (and the flipped K > rn likewise). For K = 1 the fetch is 0, which is passed to PartitionedTopKExec::try_new, whose assert!(k > 0) panics.

Expected behavior

rn < 1 / 1 > rn should return an empty result without panicking (matching the enable_window_topn = false behavior).

Additional context

The fix is to skip the rewrite when the computed fetch is 0 and let the regular FilterExec produce the empty result.