Add possibility to hide empty value in customfield group view rendering

A new parameter is defined :

```
chill_custom_fields:
    show_empty_values_in_views = true|false
```

A new method is added to CustomFieldInterface: `isEmptyValue`. To ease
the dev of new classes, an AbstractCustomField class is created, which
implements the most commons function (currently, only isEmptyValue).

A new Twig Filter is added: `chill_custom_field_is_empty`

The twig filter `chill_custom_fields_group_widget` has a new possibility
in array option : `show_empty`. Default to
chill_custom_fields.show_empty_values_in_view. May be forced by
true/false.

CustomFields/CustomFieldChoice.php

@@ -38,7 +38,7 @@ use Symfony\Component\Translation\Translator;
  * @author Julien Fastré <julien.fastre@champs-libres.coop>
  * @author Marc Ducobu <marc@champs-libes.coop>
  */
-class CustomFieldChoice implements CustomFieldInterface
+class CustomFieldChoice extends AbstractCustomField
 {
     const ALLOW_OTHER = 'other';
     const OTHER_VALUE_LABEL = 'otherValueLabel';
@@ -174,6 +174,11 @@ class CustomFieldChoice implements CustomFieldInterface
     {
         return 'Choices';
     }
+    
+    public function isEmptyValue($value, CustomField $customField)
+    {
+        return $value['_choices'] === NULL;
+    }
 
     /**
      *