Use a default font size when it can't be computed directly. (mathjax/MathJax#3458)

[dpvc]

This PR adds a default font size for the situation where the font size can't be computed, e.g, when the DOM element is not in an active DOM, or when it has display: none, rather than returning NaN.

Resolves issue mathjax/MathJax#3458.

[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.66%. Comparing base (27c47f3) to head (a39c43a).

Files with missing lines	Patch %	Lines
ts/adaptors/HTMLAdaptor.ts	63.63%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1380      +/-   ##
===========================================
- Coverage    86.66%   86.66%   -0.01%     
===========================================
  Files          338      338              
  Lines        84384    84394      +10     
  Branches      4779     3149    -1630     
===========================================
+ Hits         73131    73138       +7     
- Misses       11253    11256       +3     

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