fix(e-invoicing): cannot generate IRNs for standalone credit notes (#26824)

diff --git a/erpnext/regional/india/e_invoice/utils.py b/erpnext/regional/india/e_invoice/utils.py
index f8a230d..f497613 100644
--- a/erpnext/regional/india/e_invoice/utils.py
+++ b/erpnext/regional/india/e_invoice/utils.py
@@ -316,10 +316,6 @@
 	))
 
 def get_return_doc_reference(invoice):
-	if not invoice.return_against:
-		frappe.throw(_('For generating IRN, reference to the original invoice is mandatory for a credit note. Please set {} field to generate e-invoice.')
-			.format(frappe.bold('Return Against')), title=_('Missing Field'))
-
 	invoice_date = frappe.db.get_value('Sales Invoice', invoice.return_against, 'posting_date')
 	return frappe._dict(dict(
 		invoice_name=invoice.return_against, invoice_date=format_date(invoice_date, 'dd/mm/yyyy')
@@ -438,7 +434,7 @@
 	if invoice.is_pos and invoice.base_paid_amount:
 		payment_details = get_payment_details(invoice)
 
-	if invoice.is_return:
+	if invoice.is_return and invoice.return_against:
 		prev_doc_details = get_return_doc_reference(invoice)
 
 	if invoice.transporter and not invoice.is_return: