Sales Order Created When Shopify Order Has No Customer

## Issue:
In the Shopify integration, when an order is created in Shopify without a customer and the Ecommerce Integration settings have the default customer field empty, the system still creates a Sales Order in ERPNext.

## Steps to Reproduce:

- Create an order in Shopify without specifying a customer.

- In ERPNext, ensure the Ecommerce Integration → Shopify Settings → Default Customer field is empty.

- Observe that a Sales Order is automatically created in ERPNext.

Start by tracing the Shopify order creation path for orders without a customer, then find how an empty Default Customer setting is handled. The issue is resolved when such orders no longer create a Sales Order in ERPNext while orders with a customer or configured default continue to work.

Written by the indexing model from the issue text.