Are you sure it's not a scam?
An email from the bank - if it's not promotional - sounds fishy most of the time. It could be legit but in most cases if there is an unrecognized charge, the bank is usually interested to let you know as soon as possible - and email is not the best contact for that. In this case I'd expect the bank to call you instead of sending emails. So, don't click on any links and don't reply but call your bank back at the phone number on their website (not the email) and confirm if the charge really took place.
Should you ignore it
Although you don't care about the account, it doesn't mean you should do nothing. Even if the account is empty, if it can go into overdraft, you'll be responsible for paying it back (with penalties and interest). Of course, your bank will stop a charge they consider suspicious - but they will not stop a charge they consider legit! Talk to the bank and ask what you should do. They might be able to simply cancel your current card (if they detected a suspicious payment, they've probably suspended your card already) without issuing you a new one.
Is it OK for someone to have my card number even if they cannot use it?
Although it may sound harmless, it really is not: you'd be surprised how many online retailers permit you to make changes to your account if you provide just the name and payment information. Unfortunately, there's little you can do about that - except using your real card for online payments as little as possible (opting for virtual cards, PayPal and other payment methods instead).