Important fix to treat NULL values in conditions (Model & DSQL) correctly

In PHP `isset($row[1])` returns `false` if value in $row[1] is set, but it's NULL
(not UNDEFINED constant) and as result condition is not set at all or set
incorrectly. This bug was there long ago :)

Some test cases:
```
// field given as string or DSQL or model Field
$fields = array('name', $q->expr('now()'), $m->getElement('name'));
foreach($fields as $field) {
    $m->addCondition(array(
        // NULL value
        array($field,null),
        array($field,'=',null),
        array($field,'is',null),
        array($field,'<>',null),
        array($field,'is not',null),
        array($field,'not',null),
        // ARRAY value
        array($field,array(1,2)),
        array($field,'=',array(1,2)),
        array($field,'in',array(1,2)),
        array($field,'<>',array(1,2)),
        array($field,'not in',array(1,2)),
        array($field,'not',array(1,2)),
    ));
}
```