Fix crash when running with optimization

[QuLogic]

In that case, __doc__ is None.

[takluyver]

I think this is actually a good illustration of why we shouldn't care about the PYTHONOPTIMIZE or -O options. This line has been there with minor tweaks for over a decade (from 70be6cc), and would break as soon as you import nbconvert with optimize level 2. I can't find any issue filed about it before this one, and it looks like you've only noticed it because of a test failure on matplotlib, from an issue that went 5 years between comments.

The 'optimize' options make hardly any performance difference to normal code, anyway. If someone has a use case where they're actually useful, then they're easy enough to support, but I don't think everyone should spend time and thought on them just because they exist.

[krassowski]

If that helps someone else running tests, why not?