Fix concatenate_lists_ignore_null on rows of all_nulls

[sperlingxx]

After the rework of cudf::lists::concatenate_rows, something changed on null handling failed corresponding cuDF Java tests.
In specific, when we apply concatenate_null_policy::IGNORE, the output lists are always null free, even if input data contains rows consisting of all nulls.

In my opinion, we had better creating null mask for input rows of all_nulls, to keep align with single column concatenate.

Signed-off-by: sperlingxx [email protected]

[ttnghia]

In specific, when we apply concatenate_null_policy::IGNORE, the output lists are always null free, even if input data contains rows consisting of all nulls.

I see. I was considering this, but thinking it may be reasonable to just return an empty list instead of a null if we specify to ignore all null from concatenation, but forgot to consider the consistency with the case of concatenating 1 row. Thanks for the rework.

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[sperlingxx]

@gpucibot merge