How to overcome the incompatibiliy of `b_null` with other types in the bsoncxx library?

Question

It's my understanding that null is the state of a value, and not the type of an value within the MongoDB ecosystem. That is an element should have a key that maps to a certain type of value, and that it's possible that value is null. But the C++ driver doesn't seem to comply with this.

What is the cleanest way to handle the possibility that an element is null using the bsoncxx library? I would think that all types should be convertible to null, but I cannot figure out if or how that would be the case. This is particularly difficult in converting std::optional types into a comparable BSON type. Below is an example of some code that I would like to work, but I haven't been able to find a reasonable work around.

template<typename T>
static auto ToBsonType(const std::optional<T>& v) {
    if (v.has_value()) {
        return ToBsonType(v.value());
    } else {
        return bsoncxx::nullopt;
    }
}

My serializer works like this, which works well for all types until I get to these nullable types:

static bsoncxx::document::value ToBsonDocument(Class class) {
    auto doc = document{} 
        << "_id" << ToBsonType(class.id())
        << "a" << ToBsonType(class.a())
        << "b" << ToBsonType(class.b())
        << finalize;
    return doc;
}

Any suggestions, or do I need to rewrite the entire API if nullible types cannot be represented as a single type?