net: app: Do not try to set ANY address to interface

If the user has not specified CONFIG_NET_APP_MY_IPV4_ADDR or
CONFIG_NET_APP_MY_IPV6_ADDR, the value is set to "" in this case.
This will be converted to ANY IP address which is not useful
to be set to the network interface. So check this and just
continue the init in this case without setting the IP address.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>