[REVIEW] Handle nan values correctly in Series.one_hot_encoding

[galipremsagar]

Fixes: #7056

This PR handles nan values separately in one_hot_encoding when the given input category is None. Previously we were combining both nan & <NA> values to be the same when cat is None.

[codecov]

Codecov Report

Merging #7059 (484074a) into branch-0.18 (28d18d6) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@               Coverage Diff               @@
##           branch-0.18    #7059      +/-   ##
===============================================
+ Coverage        82.09%   82.11%   +0.01%     
===============================================
  Files               97       97              
  Lines            16477    16482       +5     
===============================================
+ Hits             13527    13534       +7     
+ Misses            2950     2948       -2     

Impacted Files	Coverage Δ
python/cudf/cudf/core/series.py	91.08% <66.66%> (-0.07%)	⬇️
python/cudf/cudf/_fuzz_testing/fuzzer.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/hash_vocab_utils.py	100.00% <0.00%> (ø)
python/cudf/cudf/core/abc.py	91.48% <0.00%> (+4.25%)	⬆️
python/cudf/cudf/utils/gpu_utils.py	58.53% <0.00%> (+4.87%)	⬆️

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