KAFKA-14900: Fix NPE in SharedServer causing flaky failure in AuthorizerTest

[gharris1727]

It appears that raftManager can become null via ControllerServer.shutdown() -> ensureNotRaftLeader() and various call sites of stop(). Rather than emit an NPE, evaluate raftManager.client while it is known to be non-null, and pass the evaluated client into the setHighWaterMarkAccessor.

I unfortunately don't have much context here, and I'm not sure if once raftManager becomes null, if it is still reasonable to evaluate highWatermark(), or whether the HighWaterMarkAccessor should return empty.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[gharris1727]

@cmccabe Could you review this fix?

[gharris1727]

This duplicates #13543 so I'll close this in favor of that more comprehensive change.

[cmccabe]

Thanks @gharris1727 .