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.
