Fix a bug in distinct: using nested nulls logic

[PointKernel]

This PR fixes a bug in cudf::distinct where the Nullate of the comparator should be determined by whether nested nulls are present.

[codecov]

Codecov Report

Merging #10848 (760c4d5) into branch-22.06 (4ad1e51) will increase coverage by 0.02%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-22.06   #10848      +/-   ##
================================================
+ Coverage         86.29%   86.32%   +0.02%     
================================================
  Files               144      144              
  Lines             22656    22656              
================================================
+ Hits              19552    19557       +5     
+ Misses             3104     3099       -5     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/numerical.py	95.88% <0.00%> (-0.30%)	⬇️
python/cudf/cudf/core/dataframe.py	93.78% <0.00%> (+0.04%)	⬆️
python/cudf/cudf/core/column/string.py	88.78% <0.00%> (+0.12%)	⬆️
python/cudf/cudf/core/groupby/groupby.py	91.79% <0.00%> (+0.22%)	⬆️
python/cudf/cudf/core/tools/datetimes.py	84.49% <0.00%> (+0.30%)	⬆️
python/cudf/cudf/core/column/lists.py	91.70% <0.00%> (+0.97%)	⬆️

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[PointKernel]

@gpucibot merge