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

.changeset/proud-turkeys-poke.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/medusa": patch
+---
+
+fix(medusa): fetching a product without a category with categories filed passed