Fix isEmpty to work with partially complete msubsup elements with empty bases. (mathjax/MathJax#3532)

[dpvc]

This PR fixes an error with the isEmpty method introduced in v4.1.1. Because superscript and subscripts are handled using an msubsup node with null entries until we know whether it will have both scripts, in the case where the base is empty, isEmpty would fail when trying to test these null children. For, example {}^2 will generate an error. This PR fixes it by checking that the child is non-null before testing it.

It may be that this wants to be a 4.1.2 release, as this is not that unusual a construction, and it causes MathJax to crash.

Resolves issue mathjax/MathJax#3532.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.49%. Comparing base (5caee44) to head (11a46f0).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1439   +/-   ##
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  Coverage    86.49%   86.49%           
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  Files          340      340           
  Lines        86044    86044           
  Branches      4833     4833           
========================================
  Hits         74425    74425           
  Misses       11595    11595           
  Partials        24       24           

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