*: check raftcluster nil (#7054)

[ti-chi-bot]

This is an automated cherry-pick of #7054

What problem does this PR solve?

Issue Number: Close #7053

What is changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• HuSharp
• lhy1024

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[codecov]

Codecov Report

Patch coverage is 40.81% of modified lines.

Files Changed	Coverage
server/cluster/cluster.go	0.00%
server/grpc_service.go	0.00%
server/server.go	45.45%
server/api/min_resolved_ts.go	50.00%
server/handler.go	50.00%

📢 Thoughts on this report? Let us know!.

[lhy1024]

/merge

[ti-chi-bot]

@lhy1024: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Details

Commit hash: 939707e