Make fences use empty <mo> for blank delimiters so the result will not be an embellished operator.  Also resolves issue #775.
Make fences use empty <mo> for blank delimiters so the result will not be an embellished operator.  Also resolves issue #775.

jinntrance/mathjax,Ivanwangcy/MathJax,minrk/mathjax-component,anusornc/MathJax,rachetfoot/MathJax,anusornc/MathJax,thomasjm/MathJax,GerHobbelt/MathJax,mcanthony/MathJax,anueshenxing/MathJax,minrk/components-mathjax,wikimedia/MathJax,cben/MathJax,nvp2211/MathJax,ManrajGrover/MathJax,ManrajGrover/MathJax,nvp2211/MathJax,jeremykenedy/MathJax,GerHobbelt/MathJax,minrk/mathjax-component,thomasjm/MathJax,jeremykenedy/MathJax,cheerfulstoic/MathJax,thomasjm/MathJax,thomasjm/MathJax,jinntrance/mathjax,anueshenxing/MathJax,Edraak/MathJax,mcanthony/MathJax,minrk/components-mathjax,cben/MathJax,perseoq/MathJax,dpvc/MathJax,thomasjm/MathJax,Ivanwangcy/MathJax,cheerfulstoic/MathJax,perseoq/MathJax,Edraak/MathJax,physikerwelt/MathJax