Checking for optional email address before sending email

Hi-

I have a form where a secondary email address is not always entered due to certain circumstances. I need to be able to send an email to the secondary email address if it is filled out. If I try to include that field in the email action and it is empty, I get an Invalid Address error from the phpmailer.php file, so I have to use a separate email action that should only be triggered if that email address exists. How do I check for it before the email action is triggered?

Thanks.