release-21.2: [CRDB-3303] ui: handle latency not defined on network page

[blathers-crl]

Backport 1/1 commits from #78292 on behalf of @Santamaura.

/cc @cockroachdb/release

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Previously when a cluster with multiple nodes had a node stopped
and then another node quickly started, the network page would crash.
This was due to the node object holding the latency value being
undefined and the ui trying to read this key when that was undefined.
This occurs when a node is in an UNAVAILABLE state.
This patch resolves the issue by being more defensive on the front
end by safely attempting to access latency and if it is undefined,
set the value to 0. The existing code is able to handle this case
afterward and will eventually set the user friendly latency status
to --.
Resolves: #59322

Release note (ui change): Fixes a bug where a node in the
UNAVAILABLE state will not have latency defined and cause the
network page to crash.

Overview list page & network page when node status is UNAVAILABLE:

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Release justification: 1 Line change for very useful bug fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[zachlite]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @koorosh)