Fix/issue mj 3453

[zorkow]

Fixes issue with sub case heuristics not considering empty rows, as reported in mathjax/MathJax#3453

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.43%. Comparing base (dceab1a) to head (431decb).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #827   +/-   ##
========================================
  Coverage    91.43%   91.43%           
========================================
  Files          165      165           
  Lines        40050    40053    +3     
  Branches      4331     4286   -45     
========================================
+ Hits         36620    36623    +3     
  Misses        3430     3430           

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