Billing validation check's Contact 1 email regardless of who owns the contribution

1.0 Problem/Motivation

Recent improvements allow us to choose a contact to connect a contribution to, instead of defaulting to Contact 1 as in the past.

2.0 Detailed steps to reproduce (embed screenshots)

* Set up a new webform with contributions enabled.
* Make a contact other than contact 1 the contribution owner.
* Make contact 1's email not required.
* Test the form, leaving contact 1's email blank.
* Submit the form. Get the error "An email address is required to complete this transaction."

3.0 Proposed resolution

PR incoming.