AggregatorFactory.getComparator() must be null-safe

[leventov]

1. Specify this in the Javadoc of this method.
2. Fix for all implementations in the repository. It's currently not null-safe at least in DoublesSketchAggregatorFactory.

@AlexanderSaydakov

[leventov]

Or, it might be much better to handle nullability centralized, not to require for each implementer of AggregatorFactory to remember about nullability. E. g. wrap with Comparator.nullsFirst(aggFactory.getComparator()) at the usage sites of getComparator(). Or add a final method getNullableComparator() to AggregatorFactory and declare protected abstract getComparator().

@nishantmonu51

[jihoonson]

it might be much better to handle nullability centralized, not to require for each implementer of AggregatorFactory to remember about nullability. E. g. wrap with Comparator.nullsFirst(aggFactory.getComparator()) at the usage sites of getComparator().

I like this idea. @nishantmonu51 any thoughts?

[gianm]

Does this need to be a release blocker for 0.14.0?

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