Fixes issue where Imputer cannot fit when there is None in a categorical or boolean column

[angela97lin]

Closes #1075

[codecov]

Codecov Report

Merging #1144 into main will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1144   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files         195      195           
  Lines       11460    11489   +29     
=======================================
+ Hits        11450    11479   +29     
  Misses         10       10           

Impacted Files	Coverage Δ
...elines/components/transformers/imputers/imputer.py	100.00% <ø> (ø)
...components/transformers/imputers/simple_imputer.py	100.00% <100.00%> (ø)
evalml/tests/component_tests/test_imputer.py	100.00% <100.00%> (ø)
...valml/tests/component_tests/test_simple_imputer.py	100.00% <100.00%> (ø)

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[freddyaboulton]

@angela97lin Looks good to me!