Fix EZP-23287: empty Image FieldType triggers twig_error_runtime

> https://jira.ez.no/browse/EZP-23287

Not sure this is the best approach, but the exception has already been
made silent by [EZP-22272](https://jira.ez.no/browse/EZP-22272).

Alternative would be to add a test in `content_fields.html.twig` but I'm
not sure either.