Partly roll back "performance upgrade"

Because isset() works faster than array_key_exists, but it's not the same. Problems start with NULL values.
More info here: http://stackoverflow.com/a/3210982/1466341

In my particular case when I use isset() then Model/DSQL generate wrong SELECTs replacing all NULL values in where() or addCondition() with some numbers, like "6". Not sure where's the problem, but better roll back this.
Merge pull request #467 from DarkSide666/patch-6

Partly roll back "performance upgrade"