How to fail the insert and update operation upon receiving blank data in a query in following scenario?

Question

I've written following two triggers on a MySQL table which will get fire before insert and update operations :

IF (NEW.full_name = '') THEN SET NEW.full_name = NEW.user_name; END IF

Now what I want to do is whenever the if condition gets satisfied then the insert/update operation should get fail.

The statement

THEN SET NEW.full_name = NEW.user_name;

should not get execute.

How should I achieve this? Please someone help me.

Thanks in advance.