fix(sales): reject sales returns where every item has zero quantity

validate_returned_items() set items_returned=True whenever a row matched
a valid item from the original document, even if its qty was 0. This let
a Sales Invoice, Delivery Note, or POS Invoice return be submitted with
every line at qty=0 - a no-op document with no stock or financial effect
that still consumed a document number and linked back to the original
transaction.

Scoped to the Sales side only: items_returned now flips to True for
Sales Invoice/Delivery Note/POS Invoice only when qty (or received_qty)
is actually negative, so an all-zero sales return correctly hits the
existing "At least one item should be entered with negative quantity"
check. Purchase Invoice, Purchase Receipt, and Subcontracting Receipt
are unchanged.

erpnext/controllers/sales_and_purchase_return.py

@@ -160,7 +160,14 @@ def validate_returned_items(doc):
 				):
 					frappe.throw(_("Warehouse is mandatory"))
 
-			if doc.doctype in ("Purchase Invoice", "Purchase Receipt", "Subcontracting Receipt"):
+			if doc.doctype in (
+				"Purchase Invoice",
+				"Purchase Receipt",
+				"Subcontracting Receipt",
+				"Sales Invoice",
+				"Delivery Note",
+				"POS Invoice",
+			):
 				if flt(d.qty) < 0 or flt(d.get("received_qty")) < 0:
 					items_returned = True
 			else: