texmath-0.13: test/writer/mml/emptyContent1.test

<<< native
[ ESub (ENumber "") (EMathOperator "") ]
>>> mml
<?xml version='1.0' ?>
<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
  <msub>
    <mn></mn>
    <mi mathvariant="normal"></mi>
  </msub>
</math>