\mhchem conversion: MathML to OOML/MathML equation leads to additional width in equation

Hello,

I am currently trying to export \mhchem equations to DocX using Pandoc. Since \mhchem is not supported natively, I transformed the TeX to MathML and then tried to convert the MathML to OOML for Word.

Using https://johnmacfarlane.net/texmath.html I could see that even in the MathML to MathML conversion the zero-width mpadded element is dropped. This would explain the additional horizontal space in the screenshot.

Looking at the OOML that Word produces directly (I am attaching the DocX file) I can see that Word added a element whereas in the the element in the OOML produced by TeXMath does not have a zero-width thus pushing the 2 down correctly but introducing white space.

Source Equation:

```
\ce{ H2O }
```

Resulting MathML (using MathJax):

```

```

DocX file as produced by Word:
[HA2O.docx](https://github.com/jgm/texmath/files/9446161/HA2O.docx)

I think that using MathML directly could open up a whole other world of equations for researchers, since we would not depend on all packages being available through Pandoc directly. I'd be happy to help and to provide further examples - unfortunately, I do not understand the code base enough to provide a fix or analysis.

Kindly
Frederik