cockroach-admin: Don't discard node status rows that are missing some fields

So, I'm totally on-board with this PR as it exists - makes sense to prevent a single parse error from stopping the whole status endpoint - but do we want to allow parsing NULL socket addresses as valid?

Put another way: It seems like "This is how CRDB works - the address field is actually an Option". Shouldn't we update our view of the struct NodeStatus to make some of these fields optional?

(basically: I think it's great to handle parse errors and continue going, but it also seems like we could / should avoid having parse errors, if we can!)

Originally posted by @smklein in https://github.com/oxidecomputer/omicron/pull/10072#discussion_r2948705482

Start by locating the struct NodeStatus and the cockroach-admin status endpoint, then inspect how NULL socket addresses and other missing fields are parsed. Determine which fields can be absent and update the relevant coverage so missing fields do not discard status rows or stop the endpoint.