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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