The driver already allows decoding BSON Null as an integer type and it should do the same for the BSON Undefined type.

This caused an error in ADL because the shell's `db.stats()` command sends the `scale` parameter as BSON undefined, which was not coerced to an integer but failed decoding instead:

> db.stats()
{
        "ok" : 0,
        "errmsg" : "failed to parse command: cannot decode undefined into an integer type, correlationID = 16025a5ab6004ec18d14584f",
        "code" : 9,
        "codeName" : "FailedToParse"
}