Fix struct scatter to correctly cascade null_mask to children columns

[ttnghia]

This fixes a bug in scatter.cuh where the struct column failed to cascade its null_mask to its children. Typically, this is automatically done during struct construction. Unfortunately, the current scatter function constructs a structs column first, then updates the parent null_mask later thus it fails to update its children's null_mask.

[codecov]

Codecov Report

Merging #8176 (0184e81) into branch-0.20 (51336df) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.20    #8176      +/-   ##
===============================================
- Coverage        82.88%   82.88%   -0.01%     
===============================================
  Files              103      104       +1     
  Lines            17668    17894     +226     
===============================================
+ Hits             14645    14832     +187     
- Misses            3023     3062      +39     

Impacted Files	Coverage Δ
python/cudf/cudf/core/tools/datetimes.py	80.42% <0.00%> (-4.11%)	⬇️
python/cudf/cudf/core/column/decimal.py	91.04% <0.00%> (-1.89%)	⬇️
python/cudf/cudf/core/column/datetime.py	88.03% <0.00%> (-1.88%)	⬇️
python/cudf/cudf/core/column/struct.py	94.73% <0.00%> (-1.56%)	⬇️
python/cudf/cudf/utils/dtypes.py	82.20% <0.00%> (-1.24%)	⬇️
python/dask_cudf/dask_cudf/groupby.py	91.28% <0.00%> (-0.88%)	⬇️
python/cudf/cudf/core/series.py	91.17% <0.00%> (-0.56%)	⬇️
python/cudf/cudf/core/index.py	92.52% <0.00%> (-0.55%)	⬇️
python/cudf/cudf/core/column/column.py	88.20% <0.00%> (-0.44%)	⬇️
python/cudf/cudf/core/column/lists.py	86.98% <0.00%> (-0.43%)	⬇️
... and 28 more

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[mythrocks]

LGTM. I have verified that this resolves #8162, and sorts out the problems I see on #8135.

[mythrocks]

@ttnghia, could we please change the PR title to be more descriptive?
"Fix child column null-masks for scattered struct columns", or something that describes what is being fixed.

[ttnghia]

@gpucibot merge