Is your feature request related to a problem?
Now Stats command has the argument bucket_nullable to control whether it should includes null buckets in group-by aggregations.

Ref #4297 and #4777, now Streamstats command will not treat null as a valid group.

Maybe we should add such a similar button to support behaviors.

Similar issue #4801

What solution would you like?
A clear and concise description of what you want to happen.

What alternatives have you considered?
A clear and concise description of any alternative solutions or features you've considered.

Do you have any additional context?
Add any other context or screenshots about the feature request here.