Conditionally include the imputer in pipelines

[eccabay]

Closes #3603

[codecov]

Codecov Report

Merging #3657 (2dc96e0) into main (8a4d0ba) will increase coverage by 16.3%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##            main   #3657      +/-   ##
========================================
+ Coverage   83.5%   99.7%   +16.3%     
========================================
  Files        335     335              
  Lines      33845   33848       +3     
========================================
+ Hits       28231   33718    +5487     
+ Misses      5614     130    -5484     

Impacted Files	Coverage Δ
...valml/tests/automl_tests/test_default_algorithm.py	100.0% <ø> (+100.0%)	⬆️
evalml/pipelines/classification_pipeline.py	100.0% <100.0%> (ø)
evalml/pipelines/utils.py	99.5% <100.0%> (+1.4%)	⬆️
evalml/tests/automl_tests/test_automl.py	99.5% <100.0%> (+99.5%)	⬆️
...ts/automl_tests/test_automl_iterative_algorithm.py	100.0% <100.0%> (+100.0%)	⬆️
evalml/tests/pipeline_tests/test_pipeline_utils.py	99.8% <100.0%> (ø)
evalml/automl/engine/engine_base.py	96.7% <0.0%> (+2.1%)	⬆️
evalml/tests/conftest.py	97.9% <0.0%> (+4.5%)	⬆️
.../preprocessing/data_splitters/time_series_split.py	96.7% <0.0%> (+13.4%)	⬆️
... and 28 more

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[jeremyliweishih]

LGTM - great work!

[chukarsten]

Great to see a performance improvement for a change! But the pessimist in me filed #3668 to investigate whether this is reverting a previous regression.