Jig, Bug fix: undefined var with chained conditions

If you're going to load some records with a condition like:
`array('( @num >= ? AND @text = ? ) OR ( @num = ? )', 2, 'bar', 3)`

it can cause some undefined var errors, when at least one record of your collection does not have one field you're using in one of your conditions.

Jig will transform this given filter array into:
`$_expr = "isset($num) && isset($text) && isset($num) && ( $num >= 2 AND $text == 'bar' ) OR ( $num == 3 )"`
which leads in an error.

With this fix, it gets correctly transformed into:
`(isset($num) && isset($text) && ( $num >= 2 AND $text == 'bar' )) OR ( isset($num) && ( $num == 3 ))`