Fix pretty-printing of coredns and nodelocaldns configmaps

[VannTen]

What type of PR is this?
/kind bug

What this PR does / why we need it:
When using

dns_upstream_forward_extra_opts:
  prefer_udp: "" # the option as no value so use empty string to just
                 # put the key

This is rendered in the dns configmap as ($ for end-of-line)

...
prefer_udp $
...

Note the trailing space.
This triggers kubernetes/kubernetes#36222,
which makes the configmap hardly readable when editing them manually or
simply putting them in a yaml file for inspection.

Trim the concatenation of option + value to get rid of any trailing
space.
Example of kubectl edit on such a configmap:

  Corefile: ".:53 {\n    errors {\n    }\n    health {\n        lameduck 5s\n    }\n
    \   rewrite name suffix <redacted> cluster.local answer auto\n    ready\n    kubernetes
    cluster.local in-addr.arpa ip6.arpa {\n      pods insecure\n      fallthrough
    in-addr.arpa ip6.arpa\n    }\n    prometheus :9153\n    forward . <redacted>
    <redacted> {\n      prefer_udp\n      max_concurrent 1000\n      prefer_udp
    \n    }\n    cache 30\n\n    loop\n    reload\n    loadbalance\n}\n"

Special notes for your reviewer:
This is pretty minor, but pretty annoying !

Does this PR introduce a user-facing change?:

Fix pretty-printing (in kubectl) of nodelocaldns and coredns configmap when using `dns_upstream_forward_extra_opts` with an empty value option.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: VannTen

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[VannTen]

/ok-to-test

[VannTen]

/cherrypick release-2.26
/cherrypick release-2.25
/cherrypick release-2.24

[k8s-infra-cherrypick-robot]

@VannTen: once the present PR merges, I will cherry-pick it on top of release-2.24, release-2.25, release-2.26 in new PRs and assign them to you.

In response to this:

/cherrypick release-2.26
/cherrypick release-2.25
/cherrypick release-2.24

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