Match Pandas logic for comparing two objects with nulls

[brandon-b-miller]

Fixes #7066

[brandon-b-miller]

So, this PR broke tests for series and dataframe mixed arithmetic, because of course comparison ops are now producing different results. However upon investigating further, I realized that dataframe/series mixed arithmetic actually isn't working at all in some situations, and this test has a grievous error (of my own making of course) where it doesn't actually compare the results properly at the end of the test, hence the failure going unnoticed.

This led to PR #7491. However, these PRs are now mutually dependent in the sense that until this PR is merged, I can't actually fix that functionality unless I spend time making the results match the logic this PR is about to change.

It doesn't make sense to me to fix broken code to produce results that will shortly thereafter be changed, forcing me to redo the code/test again.

Let me know if this seems like a sensible way to proceed.

[brandon-b-miller]

cc @cwharris @shwina

[shwina]

Synced offline and this plan seems like the least painful path forward (without having to merge 3 different PRs into a single one)

[brandon-b-miller]

@kkraus14 this is a breaking change. What due diligence should be done before merging?

[kkraus14]

@kkraus14 this is a breaking change. What due diligence should be done before merging?

We have it marked as breaking so we should be good to go. Unfortunately this may cause some breakages, but they're necessary to get the correct behavior moving forward.

[codecov]

Codecov Report

Merging #7490 (981457d) into branch-0.19 (7871e7a) will increase coverage by 0.13%.
The diff coverage is 93.60%.

@@               Coverage Diff               @@
##           branch-0.19    #7490      +/-   ##
===============================================
+ Coverage        81.86%   82.00%   +0.13%     
===============================================
  Files              101      101              
  Lines            16884    16989     +105     
===============================================
+ Hits             13822    13931     +109     
+ Misses            3062     3058       -4     

Impacted Files	Coverage Δ
python/cudf/cudf/core/index.py	92.86% <ø> (ø)
python/cudf/cudf/core/column/numerical.py	94.83% <87.50%> (-0.20%)	⬇️
python/cudf/cudf/core/column/column.py	87.77% <88.88%> (+0.01%)	⬆️
python/cudf/cudf/core/frame.py	89.00% <89.47%> (-0.02%)	⬇️
python/cudf/cudf/core/column/decimal.py	92.75% <91.30%> (-2.12%)	⬇️
python/cudf/cudf/core/dataframe.py	90.45% <95.65%> (-0.01%)	⬇️
python/cudf/cudf/core/series.py	91.25% <95.83%> (+0.47%)	⬆️
python/cudf/cudf/core/column/categorical.py	91.62% <100.00%> (+0.23%)	⬆️
python/cudf/cudf/core/column/datetime.py	89.09% <100.00%> (ø)
python/cudf/cudf/core/column/string.py	86.55% <100.00%> (+0.05%)	⬆️
... and 13 more

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[kkraus14]

@gpucibot merge