fix: skip WindowTopN rewrite when fetch is 0 to avoid PartitionedTopK panic

[viirya]

Which issue does this PR close?

• Closes enable_window_topn panics on WHERE rn < 1 (PartitionedTopK requires k > 0) #24404.

Rationale for this change

With datafusion.optimizer.enable_window_topn = true, a query filtering a partitioned ROW_NUMBER()/RANK() with rn < 1 (or the flipped 1 > rn) panics with PartitionedTopK requires k > 0.

ROW_NUMBER/RANK are always >= 1, so such a predicate matches no rows and the correct result is empty — it must not panic. The WindowTopN rule's extract_window_limit maps rn < K to a fetch of K - 1; for K = 1 that fetch is 0, which is passed to PartitionedTopKExec::try_new, whose assert!(k > 0) panics. With the optimization disabled the same query correctly returns no rows.

What changes are included in this PR?

• In WindowTopN::try_transform, bail out of the rewrite when the computed limit is 0, letting the regular FilterExec produce the (empty) result.
• Add regression cases to window_topn.slt for rn < 1, 1 > rn, and rn <= 0 (all must return empty without panicking).

Are these changes tested?

Yes — the new sqllogictest cases fail (panic) without the fix and pass with it. Existing window_topn tests continue to pass.

Are there any user-facing changes?

No API changes. rn < 1 / 1 > rn now returns an empty result instead of panicking when enable_window_topn is on, matching the behavior when it is off.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.18%. Comparing base (e7e037d) to head (0ac4c9c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24405      +/-   ##
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- Coverage   81.18%   81.18%   -0.01%     
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  Files        1110     1110              
  Lines      388906   388909       +3     
  Branches   388906   388909       +3     
==========================================
  Hits       315733   315733              
- Misses      54576    54578       +2     
- Partials    18597    18598       +1     

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[viirya]

Thanks @jayzhan211 for review!