Enable preview for empheq env

[fk128]

Related to #4349 .

From https://docs.mathjax.org/en/v3.2-latest/input/tex/extensions/empheq.html :

Note that the current implementation of the empheq environment supports only the left and right options.

MathJax fails when the box option is used, i.e. when doing

\begin{empheq}[box=\fbox]{align}
    A & = B \\
    C &= D
\end{empheq}

but not

\begin{empheq}[]{align}
    A & = B \\
    C &= D
\end{empheq}

It's not clear from the MathJax docs whether it would be possible to just ignore the option rather than fail to generate a preview.