Avoid error when \overset or \underset is empty.  Resolves isue #1474.
Avoid error when \overset or \underset is empty.  Resolves isue #1474.

dpvc/MathJax,jinntrance/mathjax,wikimedia/MathJax,GerHobbelt/MathJax,GerHobbelt/MathJax,jinntrance/mathjax