Hotfix/phone required only when physical products

[dpanta94]

All Submissions:

• [x] Does your code follow the Extendables standards?
• [ ] Have you written new tests for your changes, as applicable?
• [x] Have you successfully run tests with your changes locally?

Changes proposed in this Pull Request:

When amazon classic is used with physical products, a phone number is required to be provided. When the cart contains only virtual products the phone should not be required.

Closes #187.

How to test the changes in this Pull Request:

1. Create an order containing only physical products and one containing only virtual products.
2. In the first case, you shouldn't be able to complete checkout without providing a phone number.
3. In the second you should.

Other information:

• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changelog entry

Fix - Phone number should not be required when no physical products are present in cart.