Shopify Sync Error: BadRequest: expected String to be an id during Sales Order Sync

While attempting to sync a Shopify Sales Order, the following traceback was encountered, leading to a pyactiveresource.connection.BadRequest error:

```
File "apps/ecommerce_integrations/ecommerce_integrations/shopify/order.py", line 52, in sync_sales_order
create_items_if_not_exist(order)
File "apps/ecommerce_integrations/ecommerce_integrations/shopify/product.py", line 308, in create_items_if_not_exist
product.sync_product()
...
File "env/lib/python3.11/site-packages/pyactiveresource/connection.py", line 413, in _handle_error
raise BadRequest(err)
pyactiveresource.connection.BadRequest: Response(code=400, body="b'{"errors":{"id":"expected String to be a id"}}'", ...)
```

Error Message Summary:

`HTTP Error 400: Bad Request
{"errors":{"id":"expected String to be a id"}}
`

Possible Cause: The error seems to occur in Product.find(self.product_id) where self.product_id might not be a valid Shopify Product ID string. Shopify API expects the product ID to be a string representation of a valid ID, and passing an unexpected type (e.g., None, int, or improperly formatted value) could trigger this.

Environment:

Frappe Version: Version-15

ecommerce_integrations version = "1.20.1"

Python Version: 3.11

Shopify API Version: 2024-04

Start with sync_sales_order in apps/ecommerce_integrations/ecommerce_integrations/shopify/order.py and create_items_if_not_exist in shopify/product.py, then inspect the Product.find(self.product_id) path around the reported line. Trace the product ID value during a Shopify Sales Order sync and verify that the completed sync no longer sends an invalid ID to Shopify.

Written by the indexing model from the issue text.