JniObjWithEnv is a struct that is commonly managed by std::unique_ptr. Although it can support managing objects that are not raw pointers, any such objects have to be NullablePointers (https://en.cppreference.com/w/cpp/named_req/NullablePointer). In the past this has broken the build when compiler upgrades were carried out, so we should add in the final requirements to make the struct a true NullablePointer once and for all, to prevent similar issues from happening in the future