code for expression

I have a RumMacro that send multiple emails to all my customers about their
invoices. I need help coming up with an expression that I can build so that
it will only email the customers who are not credit released. If a customer
is credit released we enter a date into the credit released field if not we
leave it blank. At the moment all my customers( released and not released)
are getting emails I only want the ones who are not released to get the
emails. isn't there a true expression or a way that I can say if the credit
release field is empty then it is true and send them emails if it does have a
date then its false and do not email. I know there is a way I just can not
figure it out. All the help is greatly apprecitated and thanks very much.