Allow unsetting of billing address

[nicoes]

Is your feature request related to a problem? Please describe.
At the moment we are optimizing our check-out process. In this process the user has the option to choose a different billing address for the order. However, the user might also want to remove this billing address again when he/she is having second thoughts. I cannot find a way to remove this using the API. Also putting the billing address fields to null doesn't seem possible. I would like to prevent having to override the billing address with the shipping address because that would put the order in a different state than without having set the billing address.

Describe the solution you'd like
A way to unset the billing address on the setOrderBillingAddress mutation. Preferably we pass something else then CreateAddressInput. E.g. 'null' or some other type.

Describe alternatives you've considered

• Introducing an extra mutation to unset the billing address. But this solution more complex to implement in the storefront because you have to keep track of whether the billing address was previously filled out. The most simple would be: no billing address filled out = always pass 'null' to setOrderBillingAddress (or some other value).