When you had a synched Eloquent model (e.g. without changed values) but
one of those values is `null`, then that value would be considered as
dirty. `Eloquent::changed` returns false, but the value is present in
`Eloquent::get_dirty`.

This fix makes sure that a `null` value in `$attributes` is only
present in `get_dirty` when it wasn't at all *set* in `$original`.