I would do this in a duplicated comp: copy and paste the null's keyframes to the cameras, then un-parent the camera. It MIGHT work as before, but no guarantees.
That's why people started rigging cameras to nulls -- AE cameras don't behave as expected.
You could have a null follow the camera's position and rotation via expressions and bake that. But it's going to be some mildly convoluted expressions since you need to get the world position of a child.
Are you still facing this problem? If not, let us know how you solved it. If so, please let us know so we can assist you further.
Thank you for offering, Kevin. I ended up having to painstakingly set up the camera with it's own keyframes, which seems to never look as smooth as with a null, but in the end, I was able to bring my camera animation into C4D. I hope in future updates, MAXON will support translating camera animation when the camera is controlled by a null, or AE will support translating animation keys from a null to it's children.