type User struct {
    B string `bson:"b"`
    C string `bson:"c,omitempty"` 
}

func Test_UnmarshalB(t *testing.T) {
     payload := `{"c": null, "b": "b"}`
     internal := User{}
     err := bson.UnmarshalExtJSON([]byte(payload), false, &internal)
     t.Errorf("%#v %v", internal, err)
}