Use pd.Series.empty for faster check

[frances-h]

• use pd.Series.empty instead of .equals(pd.Series())

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After creating the pull request: in order to pass the changelog_updated check you will need to update the "Future Release" section of docs/source/changelog.rst to include this pull request.

[codecov-io]

Codecov Report

Merging #823 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #823      +/-   ##
==========================================
- Coverage   98.12%   98.12%   -0.01%     
==========================================
  Files         119      119              
  Lines       10877    10876       -1     
==========================================
- Hits        10673    10672       -1     
  Misses        204      204

Impacted Files	Coverage Δ
featuretools/synthesis/deep_feature_synthesis.py	97% <100%> (-0.01%)	⬇️

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[rwedge]

Looks good