Add isEmpty test (deep tree check) to MmlNode and use it to test for empty mathop.

[dpvc]

This PR adds an isEmtpy getter to the MmlNode class that does a deep tree test to see if an element contains only empty elements and that token nodes only empty text nodes as children. This is used in the test for empty \mathop elements so that it catches \mathop{\mathrm{}} as empty, the original case the Peter reported.

It also adds an additional test to check for that situation.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.51%. Comparing base (2cf7459) to head (574e1c3).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
ts/core/MmlTree/MmlNode.ts	86.66%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1428   +/-   ##
========================================
  Coverage    86.51%   86.51%           
========================================
  Files          340      340           
  Lines        86002    86032   +30     
  Branches      3186     3190    +4     
========================================
+ Hits         74406    74432   +26     
- Misses       11596    11600    +4     

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[zorkow]

lgtm.
There is one more occurrence in MathtoolsUtil.ts that you could replace.

[dpvc]

There is one more occurrence in MathtoolsUtil.ts that you could replace.

I had looked at that one, but didn't think it was necessary, but I've changed it, as I don't think it should hurt.