fix: cancel pos closing entry failure for return pos invoices (backport #47248) (#47249)

fix: cancel pos closing entry failure for return pos invoices (#47248)

(cherry picked from commit c8ee5d9a4e)

Co-authored-by: Diptanil Saha <diptanil@frappe.io>

erpnext/accounts/doctype/pos_invoice_merge_log/pos_invoice_merge_log.py

@@ -336,7 +336,7 @@ class POSInvoiceMergeLog(Document):
 		for doc in invoice_docs:
 			doc.load_from_db()
 			inv = sales_invoice
-			if doc.is_return:
+			if doc.is_return and credit_notes:
 				for key, value in credit_notes.items():
 					if doc.name in value:
 						inv = key