Fix EZP-22315: Use ez_is_field_empty to do the checks in content_fields

## Description

Link: https://jira.ez.no/browse/EZP-22315

Use the standard ez_is_field_empty to check the emptyness of content fields in the related twig template.
This way, the empty rule for a given field is coded only once.

To avoid setting each time `ez_is_field_empty( content, field.fieldDefIdentifier )`, ez_is_field_empty has been modified to also accept Fields.

## Test
Manual test and created unit test for ez_is_field_empty