So I suppose that your radio button's field has a default value ( otherwise how could it be not empty ? ).
If yes, a simple auto-enter calculation could solve your problem:
If ( IsEmpty ( Self ) ; defaultValue ; Self )
There's no default value but I see where you're going with the calc--it'll prevent a user from forcing a null into the field, a good thing; better to prevent the problem from appearing in the first place than to notify a user that there is a problem after the action has taken place!
You can use the Data box in Field Options (auto enter tab), to specify a default value for a field. The value you enter there will automatically appear in the field each time a user creates a new record.