Avoid empty "supersede domain-name-servers" directives for dhclient.conf (#10948)

Fixes https://github.com/kubernetes-sigs/kubespray/issues/10947

This patch aims to be minimal and intentionally:

- does not change the generation logic for `supersede_domain` and `supersede_search`
- does not change how `nameserverentries` (for NetworkManager) is built

It seems like `nameserverentries` in the "Generate nameservers for resolvconf, including cluster DNS"
task is built the same way as `dhclient_supersede_nameserver_entries_list`.
However, `nameserverentries` in the "Generate nameservers for resolvconf, not including cluster DNS"
task (below) is built differently for some reason. It includes `configured_nameservers` as well.
Due to these differences, I have refrained from reusing the same building logic
(`dhclient_supersede_nameserver_entries_list`) for both.

If the `configured_nameservers` addition can be removed or made to apply
to dhclient as well, we could potentially build a single list and then
generate the `nameserverentries` and `supersede_nameserver` strings from it.