App Store constantly tries to charge me for nothing
The reason why you are getting these messages when your card is empty is that they are trying to make an authorisation hold to confirm your account details. It's standard practice. If you do see these small amounts appear on your card statements, you should see a similar transaction reversing the hold a short time later.
From Apple's support website:

If you update your billing information, the iTunes Store or Mac App Store will place an authorization hold on your payment card account, usually in the amount of 1 USD or the local currency equivalent, to verify your account information.
You might see authorization requests on your online statement. These requests aren't actual charges; they are tests to confirm that your payment card account is active and has available funds to accommodate transactions. Authorization holds are removed by your financial institution shortly after your purchase clears. The time it takes to remove authorization requests varies by financial institution.