returns optional pattern

Our practice of doing null checks on fields returned by getters whose value can be null (in order to satify `mypy` linter) really feels like an anti pattern, especially when we handle null values by throwing an error

https://github.com/AllenCell/allencell-segmenter-ml/pull/519#discussion_r1779307204

Could the error just be thrown internal to the getter, for violating an invariant?

We need research into how type checking in Python should be done.