See flutter/plugins#4803 (comment) for context; originally I thought this was a case where we needed to dynamically generate the function based on specifics of the API, but it turns out that it's always unused (which means I guess I don't have warnings-as-errors turned on for ObjC like I thought I had done; I must be misremembering which languages I've done it for).

GetNullableObject was used when classes were serialized as dictionaries, but now we only use GetNullableObjectAtIndex. The only references to GetNullableObject( anywhere in the generator is where it's generated, so we can just remove it.