Inventory part with non-restricted product attributes can't be added to the cart (OCC-229)

[sarahelsaig]

Describe the bug

It's specifically only broken when the product has attributes but none are restricted to predefined values.

To Reproduce

Steps to reproduce the behavior:

1. Create a new content type.
2. Add Product part, Price part and Inventory part.
3. Add a text product attribute field (name doesn't matter).
4. Create a content item, make sure to set the inventory count to a positive number.
5. View the content item.
6. Observe that:
   • ✔️ The inventory number is as expected.
   • 🛑 The Quantity field defaults to 0 instead of 1.
7. Set the Quantity to 1 and click Checkout.
8. Observe that:
   • 🛑 An error notification saying "There are not enough X left in stock." is displayed.
   • 🛑 The product is not actually added to the cart.

Expected behavior

It looks like the inventory shopping cart event can't resolve the entry in the inventory dictionary so it defaults to assuming the inventory is 0. This should not be the case.

Screenshots

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Jira issue