Fix error in get_default_cost_center() function

While adding a new Item in Item Table within Stock Entry, threw the following error.
Ensured ".get()" in return values to gracefully avoid the error
```
  File "/home/andymax/Code/frappe-bench/apps/erpnext/erpnext/stock/doctype/stock_entry/stock_entry.py", line 425, in get_item_details
    'cost_center'			: get_default_cost_center(args, item),
  File "/home/andymax/Code/frappe-bench/apps/erpnext/erpnext/stock/get_item_details.py", line 187, in get_default_cost_center
    return (item.selling_cost_center if args.transaction_type == "selling" else item.buying_cost_center) \
 AttributeError: 'dict' object has no attribute 'transaction_type
```