Tiny improvement, do not emit text/x-math-config script section if not n...

Thank you for including the changes and the cleanup.  
After looking at your changes there was one small obvious change I saw,
moving the conditional ~@[ ... ] include of the `config` to be outside the whole <script> thing.   

This has the effect that if no `config` is specified, the whole script section will be omitted.

Kind regards,
Wim Oudshoorn.