Very important fix to treat NULL values in conditions (Model and DSQL)
correctly.

In PHP `isset($row[1])` returns `false` if value in `$row[1] = NULL`
(not UNDEFINED) and as result condition is not set at all or set
incorrectly. This bug was there long ago :)
Merge pull request #529 from DarkSide666/master

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