Fix SimpleObjectHydrator behavior when column not exists in fieldMappings, relationMappings and metaMappings

https://github.com/doctrine/dbal/commit/88c1975dda492f3dd93cddea71ebacb40ed7efa5 change seems to make unable to use ``findOneBy()`` method because of ``doctrine_rownum`` added to query.

``findOneBy()`` throws:

>Notice: Undefined index: doctrine_rownum in 
 ...\vendor\doctrine\orm\lib\Doctrine\ORM\Internal\Hydration\SimpleObjectHydrator.php line 183