If you do not employ any staff and only work with hired staff through, for example, a payroll company, your wage bill is €0. You will then automatically not receive an invoice, because there is no wage and salary bill on which premium can be calculated. If you do (have) occasionally employ someone, a wage bill will be registered and you will automatically receive an invoice.
If you have received a letter, do register via the HOP portal, even if you don't have any staff now. If you hire someone again in the future, your details will already be known.