Tax registration for German suppliers who do not have a VAT ID, but only a "Steuernummer"

Summary:

German Suppliers don't always have a VAT ID. They are only required to have a Tax Registration Number, called "Steuernummer" (in a format like 123/4567/8901). A VAT ID is only needed if they do international business.

Therefore, smaller suppliers may not have a VAT ID. When trying to create a supplier with only the Steuernummer, system will throw a validation error because it is expecting a VAT ID. So with the validation rules on, we cannot enter those suppliers (unless we would deactivate the validation).

How can we allow both types of supplier tax registrations, VAT ID as well as "Steuernummer", without giving up the validation on the VAT ID?