resolve: refuse to resolve empty hostname

resolve: refuse to resolve empty hostname

Previously, varlink or dbus methods return
io.systemd.Resolve.NoNameServers or BUS_ERROR_NO_NAME_SERVERS if an
empty hostname is provided, and thus nss-resolve returns NSS_STATUS_TRYAGAIN.

That causes getaddrinfo() returns 'Temporary failure in name resolution'
instead of 'Name or service not known'.

This makes calling varlink or dbus method with an empty hostname result
-EINVAL, and hence nss-resolve returns NSS_STATUS_NOTFOUND.

Fixes RHBZ#2039854 (https://bugzilla.redhat.com/show_bug.cgi?id=2039854).