realtek: Avoid empty lan mac in initial network setup

realtek: Avoid empty lan mac in initial network setup

If the lan_mac cannot be found, it is still used (as empty string) in
various operations. This is not valid and other 02_network scripts checking
for a non-empty string before using it. This should also be adopted for the
realtek 02_network.

Signed-off-by: Sven Eckelmann <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20241
Signed-off-by: Hauke Mehrtens <[email protected]>