fix(stock): set item_name to fetch if empty in packing slip item (#58925)

(cherry picked from commit f130c64530)

# Conflicts:
#	erpnext/stock/doctype/packing_slip_item/packing_slip_item.json

erpnext/stock/doctype/packing_slip_item/packing_slip_item.json

@@ -41,6 +41,7 @@
   },
   {
    "fetch_from": "item_code.item_name",
+   "fetch_if_empty": 1,
    "fieldname": "item_name",
    "fieldtype": "Data",
    "in_list_view": 1,
@@ -149,4 +150,4 @@
  "sort_order": "DESC",
  "states": [],
  "track_changes": 1
-}
+}