Fix issue with mphantom

[zorkow]

Fixes issue with

• mphantom elements inside other tags being spoken
• better heuristic for treating empty elements in sub/sup/over/under

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.61%. Comparing base (29ccc9a) to head (5323f1d).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #841      +/-   ##
===========================================
- Coverage    92.61%   92.61%   -0.01%     
===========================================
  Files          165      165              
  Lines        40237    40269      +32     
  Branches      4415     4391      -24     
===========================================
+ Hits         37266    37294      +28     
- Misses        2971     2975       +4     

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