fix: po, rate set as zero against the item (#18242)

diff --git a/erpnext/public/js/controllers/buying.js b/erpnext/public/js/controllers/buying.js
index 824b8d9..750900e 100644
--- a/erpnext/public/js/controllers/buying.js
+++ b/erpnext/public/js/controllers/buying.js
@@ -141,6 +141,7 @@
 
 	price_list_rate: function(doc, cdt, cdn) {
 		var item = frappe.get_doc(cdt, cdn);
+
 		frappe.model.round_floats_in(item, ["price_list_rate", "discount_percentage"]);
 
 		let item_rate = item.price_list_rate;
@@ -154,6 +155,8 @@
 
 		if (item.discount_amount) {
 			item.rate = flt((item.price_list_rate) - (item.discount_amount), precision('rate', item));
+		} else {
+			item.rate = item_rate;
 		}
 
 		this.calculate_taxes_and_totals();