network._get_lower do not cause "ls" error when there is no lower

[ilario]

On a YouHua WR1200JS I could observe two error messages from ls when running lime-config:

...
network.scandevices.owrt_network_interface_parser found ifname lo
network.scandevices.owrt_network_interface_parser found ifname br-lan
network.scandevices.owrt_network_interface_parser found ifname wan
ls: /sys/class/net/wan/lower*: No such file or directory
network.scandevices.dev_parser found WAN port wan
network.scandevices.owrt_network_interface_parser found ifname wan
ls: /sys/class/net/wan/lower*: No such file or directory
network.scandevices.dev_parser found WAN port wan
network.scandevices.owrt_device_parser found base interface not_found and derived device br-lan
network.scandevices.dev_parser got nil device
...

This happens as wan on this router is the name of the physical interface, so it does not have any lower one associated.

With the applied change, the output of lime-config does not show anymore these errors.