Crashing when using empty vec() of AsciiMath

[tsutaj]

Issue Summary

When I input vec() of AsciiMath, it causes Math Processing Error.

see also: asciimath/asciimathml#130

Steps to Reproduce:

see the link above.

Supporting information:

This issue has already fixed in the upstream branch of asciimathml (link to commit), but MathJax doesn't contain this change yet.

[dpvc]

Please see the comments there for more details.