fix(selling): make POS item-price NULL ordering match across engines (Postgres)

POS get_items keeps Item Price rows with a NULL valid_from (open-ended base
price) alongside dated rows, orders by valid_from DESC, then picks the first
matching UOM positionally via next()/[0]. MariaDB sorts NULL last for DESC, so
a dated override wins; PostgreSQL defaults to NULLS FIRST for DESC, so the
NULL-valid_from base price wins instead — the POS shows a different
price_list_rate/currency on the two engines for an item that has both an
undated standing price and a dated price.

Coalesce(valid_from, "1900-01-01") in the ORDER BY forces the NULL row to sort
last on both engines. MariaDB already placed it last for DESC, so its output is
unchanged; PostgreSQL now picks the same dated override.

erpnext/selling/page/point_of_sale/point_of_sale.py

@@ -6,6 +6,7 @@ import json
 
 import frappe
 from frappe.query_builder import Criterion, DocType, Order
+from frappe.query_builder.functions import Coalesce
 from frappe.utils import cint, get_datetime
 from frappe.utils.nestedset import get_root_of
 
@@ -231,7 +232,10 @@ def get_items(
 			.where(ItemPrice.selling == 1)
 			.where((ItemPrice.valid_from <= current_date) | (ItemPrice.valid_from.isnull()))
 			.where((ItemPrice.valid_upto >= current_date) | (ItemPrice.valid_upto.isnull()))
-			.orderby(ItemPrice.valid_from, order=Order.desc)
+			# Coalesce so a NULL valid_from (open-ended base price) sorts last under DESC on both
+			# engines: MariaDB already sorts NULL last for DESC, Postgres defaults to NULLS FIRST, which
+			# would otherwise make the base price win the positional pick over a dated override.
+			.orderby(Coalesce(ItemPrice.valid_from, "1900-01-01"), order=Order.desc)
 		).run(as_dict=True)
 
 		stock_uom_price = next((d for d in item_prices if d.get("uom") == item.stock_uom), {})