Correct on-screen kerning in math for non-mu units

Correct on-screen kerning in math for non-mu units

The current math style should have no impact on \kern1mu. This is another
benefit of a9eb87a8.

Testcase:
\begin{align*}
 & \kern1em a\\
 & {\scriptscriptstyle \kern1em a}\\
 & \kern18mu a\\
 & {\scriptscriptstyle \kern18mu a}
\end{align*}