[REVIEW] Introduce nan_as_null parameter for cudf.Index

[galipremsagar]

Fixes: #9822

This PR introduces nan_as_null parameter to cudf.Index constructor which is similar to the one present in cudf.Series constructor.

[codecov]

Codecov Report

Merging #9893 (c18d58b) into branch-22.02 (967a333) will decrease coverage by 0.08%.
The diff coverage is n/a.

❗ Current head c18d58b differs from pull request most recent head a775029. Consider uploading reports for the commit a775029 to get more accurate results

@@               Coverage Diff                @@
##           branch-22.02    #9893      +/-   ##
================================================
- Coverage         10.49%   10.40%   -0.09%     
================================================
  Files               119      119              
  Lines             20305    20501     +196     
================================================
+ Hits               2130     2134       +4     
- Misses            18175    18367     +192     

Impacted Files	Coverage Δ
python/dask_cudf/dask_cudf/sorting.py	92.30% <0.00%> (-0.61%)	⬇️
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/parquet.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/series.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/utils.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/ioutils.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/dataframe.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/multiindex.py	0.00% <0.00%> (ø)
... and 12 more

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[galipremsagar]

@gpucibot merge