fix: Use fallback conversion factor while setting incoming rate for petty purchase

- PIs for petty items (that do not need an Item record) are allowed using Item Name field
- If a different UOM is used in this case, conversion factor stays 0 and causes an error
- Fallback to 1 in `set_incoming_rate` for buying
- Selling will need a proper item, so this change is not needed there

erpnext/controllers/buying_controller.py

@@ -300,7 +300,7 @@ class BuyingController(StockController, Subcontracting):
 						raise_error_if_no_rate=False,
 					)
 
-					rate = flt(outgoing_rate * d.conversion_factor, d.precision("rate"))
+					rate = flt(outgoing_rate * (d.conversion_factor or 1), d.precision("rate"))
 				else:
 					rate = frappe.db.get_value(ref_doctype, d.get(frappe.scrub(ref_doctype)), "rate")