luci-app-vnstat2: fix configuration page when database is empty (#6746)

The output of the command "vnstat --dbiflist 1" is empty when there are
no interfaces in the database. Add a check to avoid a "result.stdout is
undefined" error in that case.

Ref: https://forum.openwrt.org/t/error-in-luci-vnstat2-configuration-menu/179700
Fixes: 3ac4f56 ("luci-app-vnstat2: use more efficient way to get database interfaces")

Signed-off-by: Jan Hoffmann <jan@3e8.eu>

applications/luci-app-vnstat2/htdocs/luci-static/resources/view/vnstat2/config.js

@@ -60,7 +60,7 @@ return view.extend({
 		o.load = function(section_id) {
 			return fs.exec('/usr/bin/vnstat', ['--dbiflist', '1']).then(L.bind(function(result) {
 				var databaseInterfaces = [];
-				if (result.code == 0) {
+				if (result.code == 0 && result.stdout) {
 					databaseInterfaces = result.stdout.trim().split('\n');
 				}