[TASK][JNI] Investigate train of `null_count` after `explode`
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Description
While analyzing an nsys trace for a Spark job with deeply nested tables, we see an `explode` kernel call that is followed by a train of `null_count`, which end in `is_valid`.
After we call `cudf::explode` we build up a table, and construct java `ColumnVector` objects. I think the construction of these objects is triggering it.
This task is to confirm that the columns with missing a null count are coming from the `explode` kernels. If they are coming from `explode`, it would be great if `explode` could compute null count as part of that kernel.
In this screenshot, it is the ~20ms at the end after `explode`:
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