Handle crashed member in quorum queue status (backport #15003)

[mergify]

Proposed Changes

ra:key_metrics/1 can return a very small metrics map in case a member is dead and it terminated so early that it hasn't recorded any counters yet.

In this edge case the quorum_status command used to crash, although returning status of other members is still useful.

$ rabbitmq-queues quorum_status <queue>
Status of quorum queue <queue> on node rabbit@<host> ...
Error:
{{:case_clause, %{state: :noproc, membership: :unknown, machine_version: 6}}, [{:rabbit_quorum_queue, :"-status/2-lc$^0/1-0-", 2, [file: ~c"src/rabbit_quorum_queue.erl", line: 1252]}, {:rabbit_quorum_queue, :"-status/2-lc$^0/1-0-", 2, [file: ~c"src/rabbit_quorum_queue.erl", line: 1311]}, {:rabbit_quorum_queue, :status, 2, []}]}

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Further Comments

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This is an automatic backport of pull request #15003 done by [Mergify](https://mergify.com).