Blank billing address created on paid event registrations with no billing address

If you use an 'external' payment provider;
And you don't have any 'billing' address fields on your event registration form;
Then a blank 'billing' address is created for the registering contact.

I've tracked this down to the murky depths of CRM_Event_Form_Registration_Confirm::fixLocationFields

I'm not sure what this function is doing. I need to investigate more. Not sure why it's called on every field in the params either.

This is major rather than minor because of the undocumented complexities in the class.

Proposed fix is: if we don't ask for billing details on the form, then we don't want to store them in Civi. So don't create a blank billing address - even if we're taking a payment.