https://bitbucket.org/eactive/integrated-content-bundle/src/7bf96a332fdcebf3c0ae460b97a331324eede519/Document/Content/ContentRepository.php?at=0.5&fileviewer=file-view-default#ContentRepository.php-25
--> The relation id is always used, also when null.
https://bitbucket.org/eactive/integrated-content-bundle/src/7bf96a332fdcebf3c0ae460b97a331324eede519/Document/Content/ContentRepository.php?at=0.5&fileviewer=file-view-default#ContentRepository.php-27
--> on $relation check on this line should not be there