Model: "dirty" fix nr.3

Yes I know, I know ...
There still was problem with situations when:
1) $this->data itself is null !? Can't explain why this happens - probably some ATK4 magic :)
2) $this->data[$name] is initialized, but it's value is NULL. As result isset($this->data[$name]) still return false if value of array element is NULL.

well - this change fix both cases.