The semantics of the API contract is distinct from its implementation details (lazy loading).
Treating null and undefined as distinct is never a requirement for general-purpose API design. That is, there is always an alternative design that doesn't rely on that misfeature.
As for patches, while it might be true that JSON Merge Patch assigns different semantics to null and undefined values, JSON Merge Patch is a worse version of JSON Patch, which doesn't have that problem, because like I originally described, the semantics are explicit in the data structure itself. This is a transformation that you can always apply.