refactor: add guard clause in for loop

Reduce overly indented code/improve readability.

erpnext/controllers/accounts_controller.py

@@ -986,14 +986,17 @@ class AccountsController(TransactionBase):
 		total_overbilled_amt = 0.0
 
 		for item in self.get("items"):
-			if item.get(item_ref_dn):
+			if not item.get(item_ref_dn):
+				continue
+
 			ref_amt = flt(frappe.db.get_value(ref_dt + " Item",
 				item.get(item_ref_dn), based_on), self.precision(based_on, item))
 			if not ref_amt:
 				frappe.msgprint(
 					_("Warning: System will not check overbilling since amount for Item {0} in {1} is zero")
 						.format(item.item_code, ref_dt))
-				else:
+				continue
+
 			already_billed = frappe.db.sql("""
 				select sum(%s)
 				from `tab%s`