properly simulate `IS NULL`

[staabm]

instead of

SELECT `adp`.*, `ada`.`spracheid`
                FROM `adp`
                INNER JOIN `ada` ON ada.adaid = adp.adaid
                WHERE (adpid = NULL)
                LIMIT 1

we should simulate

SELECT `adp`.*, `ada`.`spracheid`
                FROM `adp`
                INNER JOIN `ada` ON ada.adaid = adp.adaid
                WHERE adpid IS NULL
                LIMIT 1

for code like

            $query = '
                SELECT `adp`.*, `ada`.`spracheid`
                FROM `adp`
                INNER JOIN `ada` ON ada.adaid = adp.adaid
                WHERE (adpid = ?)
                LIMIT 1';

            $adpMapper = new Application_Model_Mapper_Adp();
            $adpModel = $adpMapper->fetchRowByStatement(new ClxProductNet_DbStatement($query, [$adpid]));

[staabm]

Related https://stackoverflow.com/a/10340795

[staabm]

The actual query simulation might be unnecessary.. but we could determine non-nullable types, when IS NULL is used in WHERE

[hemberger]

This is partially completed by #596. We still need to support the WHERE x=NULL case.