Allow null values in TaxonSelectionToCollectionTransformer

In our project we have:

```diff
diff --git a/src/Sylius/Bundle/TaxonomiesBundle/Form/Type/TaxonSelectionType.php b/src/Sylius/Bundle/TaxonomiesBundle/Form/Type/TaxonSelectionType.php
index a9dcb36..9c77828 100644
--- a/src/Sylius/Bundle/TaxonomiesBundle/Form/Type/TaxonSelectionType.php
+++ b/src/Sylius/Bundle/TaxonomiesBundle/Form/Type/TaxonSelectionType.php
@@ -70,6 +70,7 @@ class TaxonSelectionType extends AbstractType
             $builder->add($taxonomy->getId(), 'choice', array(
                 'choice_list' => new ObjectChoiceList($this->taxonRepository->getTaxonsAsList($taxonomy)),
                 'multiple'    => $options['multiple'],
+                'empty_value' => '---',
                 'label'       => /** @Ignore */ $taxonomy->getName()
             ));
         }
```

This causes `ContextErrorException: Warning: Invalid argument supplied for foreach() in src/Sylius/Bundle/TaxonomiesBundle/Form/DataTransformer/TaxonSelectionToCollectionTransformer.php line 83`.