Null bson

The goal is to make sure all bson are null (and correctly null, in some part of the code, we can see bson b = { 0, NULL }, which is not enough).
Once we are sure all bson are correctly at the beginning, I will try to make sure we destroy them correctly.

For example no data is allocated when calling bson_empty(&b), bson_init_data(&b, mydata). This is a problem because we don't know when to call bson_destroy or not...