Fix handling of all-whitespace strings in `MeanCharactersPerSentence`

[sbadithe]

• closes #232

[codecov[bot]]

Codecov Report

Merging #234 (53b9df7) into main (b83c5ad) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #234      +/-   ##
==========================================
- Coverage   98.54%   98.53%   -0.01%     
==========================================
  Files          27       27              
  Lines         685      683       -2     
==========================================
- Hits          675      673       -2     
  Misses         10       10              

Impacted Files	Coverage Δ
nlp_primitives/mean_characters_per_sentence.py	100.00% <100.00%> (ø)
...mitives/tests/test_mean_characters_per_sentence.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.