fix(core-flows): unsafe access to a variant property (#11588)

**What**
- fix unsafe access `manage_inventory` flag if variant doesn't exist for the item

---

CLOSES https://github.com/medusajs/medusa/issues/11587

packages/core/core-flows/src/cart/workflows/list-shipping-options-for-cart.ts

@@ -172,7 +172,7 @@ export const listShippingOptionsForCartWorkflow = createWorkflow(
 
           const itemsAtLocationWithoutAvailableQuantity = cart.items.filter(
             (item) => {
-              if (!item.variant.manage_inventory) {
+              if (!item.variant?.manage_inventory) {
                 return false
               }