There is a bug in the C driver where it appends undefined instead of null in bson_append_iter (https://github.com/mongodb/libbson/blob/master/src/bson/bson.c#L1245-L1247):

case BSON_TYPE_NULL:
   ret = bson_append_undefined (bson, key, key_length);
   break;

A similar append in bson_append_value at https://github.com/mongodb/libbson/blob/master/src/bson/bson.c#L1723-L1725 seems to be correctly implemented.