ez_is_field_empty does not work for a "date and time" field

If you have an attribute of the "Date and time" datatype with an identifier "end_date", this check always returns "false".

Steps to reproduce:

1. Add a "date and time" attribute to your article class
2. Create a new article, and leave the "date and time" empty
3. Add {{ ez_is_field_empty( content, 'end_date' ) }} to your vendor/ezsystems/demobundle/EzSystems/DemoBundle/Resources/views/full/article.html.twig template, considering that "end_date" is the alias of your date time attribute
4. Navigate to your article from the frontend (eZ Demo)

It should return "1", but nothing is returned.
Everything work correctly with the "date" attribute.