Update design of content field value in form field type:
Currently the user doesn't have more clues than an empty block to know if form field type in content view is empty (see attachment "current-form-field.png").

Purpose of this ticket is to harmonize the content view for form field type when it is empty with the rest of field types in this specific case:

• Add text letting the user know that that field is empty, like it currently happens in the other field types with ez-content-field-value with <em>This field is empty</em>

• Check if height of iframe when form field type is empty (currently set is 150px) could be the same as the rest of field types when they are empty;
• Check if background color could be changed to $ez-white too.