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: 3ac4f567

 ("luci-app-vnstat2: use more efficient way to get database interfaces")

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