Some curve objects don't have an evaluated mesh at all, but line art
currently assumes that all curve objects have one before converting
it to a mesh internally. Fix this by checking if the curve object has an
evaluated mesh before skipping it.
The remaining problem is that evalauted from non-mesh objects or
evaluated curves from non-curve objects, etc. will be ignored if
"Allow Duplicates" is off. That's a different problem though.
Differential Revision: https://developer.blender.org/D14036
88ff5e5fb9