Adds drop null columns preprocessing component

[angela97lin]

Closes #833

[codecov]

Codecov Report

Merging #834 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #834   +/-   ##
=======================================
  Coverage   99.67%   99.67%           
=======================================
  Files         188      191    +3     
  Lines        7338     7428   +90     
=======================================
+ Hits         7314     7404   +90     
  Misses         24       24           

Impacted Files	Coverage Δ
evalml/pipelines/components/__init__.py	100.00% <ø> (ø)
...alml/pipelines/components/transformers/__init__.py	100.00% <100.00%> (ø)
.../components/transformers/preprocessing/__init__.py	100.00% <100.00%> (ø)
...ts/transformers/preprocessing/drop_null_columns.py	100.00% <100.00%> (ø)
...ponent_tests/test_drop_null_columns_transformer.py	100.00% <100.00%> (ø)

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[kmax12]

did a quick pass. should get one more approval before merging

[dsherry]

Unit tests look great! I left a couple questions, one about making _cols_to_drop private and another about the boundary condition. But this looks ready to merge! We can finish those discussions and update if necessary.