fix: ignore item-less maintenance visit for sr no (#30684)

diff --git a/erpnext/maintenance/doctype/maintenance_visit/maintenance_visit.js b/erpnext/maintenance/doctype/maintenance_visit/maintenance_visit.js
index 72686e7..e2f6cb3 100644
--- a/erpnext/maintenance/doctype/maintenance_visit/maintenance_visit.js
+++ b/erpnext/maintenance/doctype/maintenance_visit/maintenance_visit.js
@@ -12,6 +12,9 @@
 		// filters for serial no based on item code
 		if (frm.doc.maintenance_type === "Scheduled") {
 			let item_code = frm.doc.purposes[0].item_code;
+			if (!item_code) {
+				return;
+			}
 			frappe.call({
 				method: "erpnext.maintenance.doctype.maintenance_schedule.maintenance_schedule.get_serial_nos_from_schedule",
 				args: {