I don't have a field for no previous expiry date.
You don't need a field for it. It's in the _cbsubs_subscriptions table.
I did try this would looks like it should work, but doesn't
You need to check for NULL. Try changing the below.
From:
Code:
sub.`previous_expiry_date` = ''
To:
Code:
sub.`previous_expiry_date` IS NULL