fix: sliding window min() returns wrong value for all-NULL windows

[neilconway]

Which issue does this PR close?

• Closes Sliding window min() returns stale value when all non-NULL values leave the frame #23872

Rationale for this change

SlidingMinAccumulator::update_batch skipped NULL values. This meant that if all the non-NULL values in a window frame were retracted, the window frame would not be empty but the MovingMin data structure by the SlidingMinAccumulator would not contain any values. This resulted in incorrectly returning a stale non-NULL value for a sliding min() over a window frame consisting of only NULL values.

Along the way, optimize the min and max sliding window accumulators to make them both more efficient and more symmetric with one another. In the original coding, SlidingMaxAccumulator included NULL values but SlidingMinAccumulator omitted them, in part because omitting NULLs made the original retract_batch implementation more expensive. This PR optimizes retract_batch, so we can now use the same scheme for both the min and max sliding accumulators:

• Omit NULLs on update_batch (this improves on the prior behavior of max)
• Efficiently account for NULLs in retract_batch (this improves on the prior behavior of min)
• Ensure correct results for all-NULL window frames (this fixes the prior bug in min).

What changes are included in this PR?

• Fix bug in min() over all-NULL window frames
• Optimize SlidingMinAccumulator::retract_batch (avoid materializing values just to count NULLs)
• Optimize SlidingMinAccumulator::update_batch (omit NULLs), also improving symmetry with min
• Optimize both accumulators to stop caching the current min / max; this saves a few clones, but perhaps more importantly it is simpler and avoids the risk of inconsistency between the cached value and the underlying MovingMin / MovingMax data structure

Are these changes tested?

Yes, new tests added.

Are there any user-facing changes?

No, aside from the bug fix.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 59.32203% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.69%. Comparing base (16471ee) to head (0dd135d).

Files with missing lines	Patch %	Lines
datafusion/functions-aggregate/src/min_max.rs	59.32%	4 Missing and 20 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23874      +/-   ##
==========================================
- Coverage   80.69%   80.69%   -0.01%     
==========================================
  Files        1090     1090              
  Lines      370357   370389      +32     
  Branches   370357   370389      +32     
==========================================
+ Hits       298859   298873      +14     
  Misses      53684    53684              
- Partials    17814    17832      +18     

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