fix(medusa): fetching a product without a category with categories filed passed (#13020)

**What**
- adding `categories` to `fields` param would return 404 for an existing product if the product was not associated with a category

---

CLOSES CMRC-1046

packages/modules/product/src/services/product-module-service.ts

@@ -225,6 +225,7 @@ export default class ProductModuleService
       this.getProductFindConfig_(config),
       sharedContext
     )
+
     const serializedProducts = await this.baseRepository_.serialize<
       ProductTypes.ProductDTO[]
     >(products)