Fix EZP-20867: Error when creating Image fieldtype without validator configuration

https://jira.ez.no/browse/EZP-20867

Similar approach used in: https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/Core/FieldType/TextLine/Type.php#L48
Merge pull request #361 from fdob/patch-1

Fix EZP-20867: Error when creating Image fieldtype without validator configuration