statefiles: don't write empty hosts files

statefiles: don't write empty hosts files

This is mostly cosmetic, skip writing out hosts files for interfaces
where it makes little sense to do so (i.e. where the hosts file will
anyway be empty).

Closes: https://github.com/openwrt/odhcpd/issues/338
Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/342
Signed-off-by: Álvaro Fernández Rojas <[email protected]>