mwan3: handle missing interface gracefully

mwan3: handle missing interface gracefully

`ubus.call(sprintf('network.interface.%s', ifname), 'status', {})`
returns null if the interface `ifname` doesn't exists (yet).

For pppoe interfaces using `option ipv6 auto`, a virtual interface suffixed `_6`
is automatically created once the connection is established,
but until then it doesn't exists.

Fixes: 6423781254b9f3e52c6102fb2cbcd9f99f2445a3 ("mwan3: reimplement rpcd plugin using ucode")
Signed-off-by: Etienne Champetier <redacted>