Change namespace contains_nulls to struct

[davidwendt]

This PR just changes the contains_nulls namespace to be struct instead in order to remove it from the namespace list in the libcudf doxygen documentation.

Changing this to struct does not require any other code changes.

[codecov]

Codecov Report

Merging #8523 (2648284) into branch-21.08 (93ce6c7) will increase coverage by 0.40%.
The diff coverage is 100.00%.

❗ Current head 2648284 differs from pull request most recent head 76f4fb2. Consider uploading reports for the commit 76f4fb2 to get more accurate results

@@               Coverage Diff                @@
##           branch-21.08    #8523      +/-   ##
================================================
+ Coverage         82.57%   82.98%   +0.40%     
================================================
  Files               109      109              
  Lines             17870    18197     +327     
================================================
+ Hits              14757    15100     +343     
+ Misses             3113     3097      -16     

Impacted Files	Coverage Δ
python/cudf/cudf/core/frame.py	93.41% <ø> (+0.45%)	⬆️
python/cudf/cudf/core/column/column.py	87.64% <100.00%> (+0.11%)	⬆️
python/cudf/cudf/core/dataframe.py	90.98% <100.00%> (+0.33%)	⬆️
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/cudf/cudf/_fuzz_testing/io.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/applyutils.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_csv.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_orc.py	100.00% <0.00%> (ø)
... and 45 more

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[trxcllnt]

@gpucibot merge