Fix for empty superscript base

[orangemug]

When converting a superscript without a base renders incorrectly in word

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block" class="tml-display" style="display:block math;">
  <msup>
    <mn></mn>
    <mn>4</mn>
  </msup>
</math>

Renders like this

Whereas it should render like this

This is because empty text nodes get rendered as empty boxes

<m:t xml:space="preserve"></m:t>

I've instead replaced that with ZERO WIDTH SPACE (​)

<m:t xml:space="preserve">&#x200B;</m:t>