Add default value to product item category in case none is provided

[bmigirl]

I want to merge this change because it fixes issue when searching for a product that has no category and the app crashes

SALEOR-1398

Screenshots

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. The changes are tested.
4. The code is documented (docstrings, project documentation).
5. Changes are mentioned in the changelog.

Test Environment Config

API_URI=https://master.staging.saleor.rocks/graphql/

[dominik-zeglen]

Could you add story for this case?

[patrys]

Task linked: SALEOR-1398 Storefront crashes on searching for e.g. "AA2" for logged in admin user