avoid errors when binding empty propel collection to a symfony2 form

PropelCollectionType.php

<?php
        $callback = function($data = null) {
            if (empty($data)) {
                return new \PropelObjectCollection();
            }

            return $data;
        };
        $builder
            ->appendClientTransformer(new CallbackTransformer($callback, $callback));
        ;

interesting, I've maybe done something wrong. I'l try to dig into that.