ROC Curve Refactor for Nullable Types

[chukarsten]

Fixes #3471

[codecov]

Codecov Report

Merging #3465 (c60164f) into main (882a9e4) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3465     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        336     336             
  Lines      33364   33400     +36     
=======================================
+ Hits       33238   33274     +36     
  Misses       126     126             

Impacted Files	Coverage Δ
evalml/model_understanding/metrics.py	100.0% <100.0%> (ø)
evalml/tests/conftest.py	97.5% <100.0%> (+0.1%)	⬆️
...ml/tests/model_understanding_tests/test_metrics.py	100.0% <100.0%> (ø)

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[jeremyliweishih]

beautiful work 🤌