Hi - i tried this but its does not work. It does not recognise that it is not empty. The Action is in a workflow with other actions. When I just create a WF with just this action it works perfectly.
if the action work on its own there is not a problem with that specific action.
if it doesn't work in context of your workflow there must be other dependencies/reasons why it doesn't work. maybe action is executed too late - when the field is already populated.
but without knowing specifics of your workflow it's hard to guess.