bug description :
> if you change the options of a custom field choice (i.e. from a
> multiple to a single, removing or adding allow_other), the data
> representation change and do not match with the expected
> representation of the form.
This commit fix this bug by switching the data representation to the
current options.
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.
