Fix problem with MathJax reference in liteDOM

[dpvc]

This PR fixes a problem with the changes in b7f1942 that cause an error in node applications that use direct loading of the MathJax modules, where the MathJax variable is not defined. I thought the MathJax?.loader would check for that, but it turns out that isn't the case, and a more explicit check for being defined is needed.

[dpvc]

This was part of my testing to see that wicked-good-expath is not needed in node applications that use SRE, and with this fix, everything looks good to remove that dependency.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.68%. Comparing base (351698e) to head (15aa49a).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
ts/adaptors/lite/Element.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1280   +/-   ##
========================================
  Coverage    86.68%   86.68%           
========================================
  Files          337      337           
  Lines        84105    84105           
  Branches      4750     4750           
========================================
  Hits         72909    72909           
  Misses       11173    11173           
  Partials        23       23           

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[zorkow]

looks good, although the tests are complaining about the new line not being covered.

[dpvc]

the tests are complaining about the new line not being covered.

That file never had complete coverage. We only have complete coverage for the TeX input jax and the ts/util folder. Everything else still needs work.