Fix EZP-21089: Creating an article with public api throw warning on xmltext in reagards to relations

Warning: array_flip(): Can only flip STRING and INTEGER values! in /var/www/apache2php53/ezp5/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/Repository/RelationProcessor.php on line 108

Issue: https://jira.ez.no/browse/EZP-21089