fix(math): Fractions must have padding to avoid visual confusion

[Omikhleia]

MathML Core 3.3.2: "To avoid visual confusion between the fraction bar and another adjacent items (e.g. minus sign or another fraction's bar)"

Closes #1801

[Omikhleia]

Impacted tests:

• tests/math-bigops.xml
• tests/math-fractions.xml
• tests/math-macros.sil
• tests/math-stretchy.xml

--> Checking them atm.

[Omikhleia]

--> Checking them atm.

They all look good to me, the small extra padding is not too visible but fixes the issue:
For the record tests/math-fractions.xml experienced it, in its last test, i.e.

<math>
  \frac{i+1}{}
  \frac{}{i+1}
</math>

This was producing a single long continuous fraction bar.
Which might have been the wanted effect here? But this is not the correct way to handle alignments anyway, and would have been confusing without empty denominator/numerator...

Before/After:

Now I'd need to push the fixed test expectations.... And still don't have the correct version of Gentium(?) in my dockers...