Clean up convoluted null checks in VehicleFactory

[CarlinLiao]

https://github.com/spartalab/aimsim/blob/56e9beaeb158b0c045f117e3b44f2c7421879659/aimsim/endpoints/factories/factory.py#L73-L74

From #2 (comment)

@pyrito

These "null" checks seems a little convoluted, they work but they could be written more cleanly maybe? Not fully sure.

You could probably write a function for some of these null check stuff too, or find some way to make it clean.

@CarlinLiao

That's possible but it's just two null checks so I'm not super concerned about it.