Indicator of ressource assignment

Hello all, 

I am new to this forum.

I would like to create in the table of my Gantt Chart an indicator showing if one or more resource is assigned to the task (ie green light if any resource, red if none).

i am trying to use a custom field indicator but it does not work.

Exemple : i am trying to check if the field [Resource name] is Null or not.

Any idea how to do ? 

Thanks.