Add support for `IS NOT NULL` for SQL based datasouces, close #924

Add "out of the box" support for:
```php
$conditions = array('fieldname' => array('IS NOT' => null));
```
and for consistency:
```php
$conditions = array('fieldname' => array('IS' => null));
```
which is similar as:
```php
$conditions = array('fieldname' => null);
```