Github user koertkuipers commented on the issue: https://github.com/apache/spark/pull/14222 there is a usefulness to this `ReduceAggregator` beyond `.reduceGroups`. basically you can take any Aggregator without a zero and turn it into a valid Aggregator, with the caveat being that the result is nullable and will be null if no inputs are provided to the Aggregator.
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