fix(india): keyerror while generating e-way bill from an e-invoice (#30879)

diff --git a/erpnext/regional/india/e_invoice/utils.py b/erpnext/regional/india/e_invoice/utils.py
index df3981a..cfe9cee 100644
--- a/erpnext/regional/india/e_invoice/utils.py
+++ b/erpnext/regional/india/e_invoice/utils.py
@@ -443,7 +443,7 @@
 		dict(
 			gstin=invoice.gst_transporter_id,
 			name=invoice.transporter_name,
-			mode_of_transport=mode_of_transport[invoice.mode_of_transport],
+			mode_of_transport=mode_of_transport[invoice.mode_of_transport or ""] or None,
 			distance=invoice.distance or 0,
 			document_name=invoice.lr_no,
 			document_date=format_date(invoice.lr_date, "dd/mm/yyyy"),