fix(kernel-network-modules): allow specifying empty --hostonly-nics

fix(kernel-network-modules): allow specifying empty --hostonly-nics

Commit 1e92f728 ("Add a --hostonly-nics option") allows only installing
the network drivers for specified NICs. But it doesn't allow an emtpy
--hostonly-nics. Specifying empty hostonly-nics is needed to address the
following use cases,
 - drivers/s390/net is installed regardless of --hostonly-nics. There is
   no need to repeat it.
 - get_dev_module couldn't find the driver for a bond or a bridge
   network interface so it's better for a module to install a bond
   or bridge driver itself
 - dracut kdump module wants to install the needed network drivers by
   itself because it can't add the --hostonly-nics option to dracut when
   itself is a dracut module

This patch distinguishes between an empty --hostonly-nics and --hostonly-nics
not added so a user can pass an empty --hostonly-nics option.