[dpvc]

This PR causes the key explorer to skip nodes with empty speech (or speech set to "none") and also prevents the subtitles from showing the "none" character (just in case). This avoids a situation where the explorer moves to a node with no speech, which can be confusing when nothing is spoken.

Resolves issue mathjax/MathJax#3618.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.13%. Comparing base (e521742) to head (da04dfd).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1581     +/-   ##
==========================================
  Coverage    87.13%   87.13%             
==========================================
  Files          392      392             
  Lines        89279    89280      +1     
  Branches      5079     3386   -1693     
==========================================
+ Hits         77793    77794      +1     
  Misses       11486    11486             

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[zorkow]

One suggestion of a refactor. O/w works fine.