I'm trying to write a formula that looks at a date field (Termination_Date__c) and if the field already has a date then do nothing but if it is blank then enter today's date.  I assume I would do something like this:

IF(ISBLANK(Termination_Date__c),TODAY(),---but here in the FALSE return area what do I enter to do nothing if the field Termination_Date__c is not blank?---)

I'm not sure how to proceed.  Any help would be appreciated.