Handle `None` in `script::devtools::determine_auto_margins`

[jonleighton]

See @emilio's comment:

I just noticed this... This unwrap may not be safe, I don't think anything guarantees that the element is not in a display: none subtree... Though it was a pre-existing bug, and I don't know where this code runs.

It remains to be seen if the unwrap is actually a problem, but from a cursory look at the code I think it probably is. Probably inspecting a node via devtools which is in a display: none subtree will trigger the problem.

[highfive]

cc @fitzgen