Backport of cni: fix regression in falling back to DNS owned by dockerd into release/1.6.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #20189 to be assessed for backporting due to the inclusion of the label backport/1.6.x.

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The below text is copied from the body of the original PR.

---

In #20007 we fixed a bug where the DNS configuration set by CNI plugins was not threaded through to the task configuration. This resulted in a regression where a DNS override set by dockerd was not respected for bridge mode networking. Our existing handling of CNI DNS incorrectly assumed that the DNS field would be empty, when in fact it contains a single empty DNS struct.

Handle this case correctly by checking whether the DNS struct we get back from CNI has any nameservers, and ignore it if it doesn't. Expand test coverage of this case.

Fixes: #20174

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In addition to the unit tests here, I verified this works with the Docker DNS override. Using the default values for dockerd and this patch, I get the following in a Nomad job using mode="bridge" networking:

$ docker exec -it 8b38 cat /etc/resolv.conf
# Generated by Docker Engine.
# This file can be edited; Docker Engine will not make further changes once it
# has been modified.

nameserver 208.67.222.222
nameserver 208.67.220.220
nameserver 192.168.1.1
search mynetworksettings.com

# Based on host file: '/run/systemd/resolve/resolv.conf' (legacy)
# Overrides: []

And after setting an override of --dns=8.8.8.8 in my docker.service unit, I get the following for the same job:

$ docker exec -it ec42 cat /etc/resolv.conf
# Generated by Docker Engine.
# This file can be edited; Docker Engine will not make further changes once it
# has been modified.

nameserver 8.8.8.8
search mynetworksettings.com

# Based on host file: '/run/systemd/resolve/resolv.conf' (legacy)
# Overrides: [nameservers]

---

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