Avoid boxing/unboxing in PostAggregators

[nishantmonu51]

Review comment -
#5958 (comment)
It looks that we need specialized PostAggregator for primitive types and a new method isNull() to avoid boxing/unboxing.

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This issue has been marked as stale due to 280 days of inactivity.
It will be closed in 4 weeks if no further activity occurs. If this issue is still
relevant, please simply write any comment. Even if closed, you can still revive the
issue at any time or discuss it on the dev@druid.apache.org list.
Thank you for your contributions.