Fixes summary for Pipelines without estimators

[angela97lin]

Closes #706

[codecov]

Codecov Report

Merging #707 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #707      +/-   ##
==========================================
+ Coverage   99.19%   99.21%   +0.02%     
==========================================
  Files         140      140              
  Lines        4952     4985      +33     
==========================================
+ Hits         4912     4946      +34     
+ Misses         40       39       -1     

Impacted Files	Coverage Δ
evalml/pipelines/pipeline_base.py	99.51% <100.00%> (+0.47%)	⬆️
...ion_pipeline_tests/test_catboost_classification.py	100.00% <100.00%> (ø)
...ication_pipeline_tests/test_logistic_regression.py	100.00% <100.00%> (ø)
...ine_tests/classification_pipeline_tests/test_rf.py	100.00% <100.00%> (ø)
...tion_pipeline_tests/test_xgboost_classification.py	100.00% <100.00%> (ø)
...ression_pipeline_tests/test_catboost_regression.py	100.00% <100.00%> (ø)
...egression_pipeline_tests/test_linear_regression.py	100.00% <100.00%> (ø)
...ts/regression_pipeline_tests/test_rf_regression.py	100.00% <100.00%> (ø)
...gression_pipeline_tests/test_xgboost_regression.py	100.00% <100.00%> (ø)
evalml/tests/pipeline_tests/test_pipelines.py	99.63% <100.00%> (+0.01%)	⬆️

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[dsherry]

Looking good, left suggestions

[dsherry]

Nice going! Left a couple style suggestions on the impl but lgtm 🚢