client: avoid panic when leader gRPC conn is nil

[CabinfeverB]

What problem does this PR solve?

Issue Number: close #7910

What is changed and how does it work?

Check List

Tests

• Unit test

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• HuSharp
• rleungx

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[CabinfeverB]

/merge

[ti-chi-bot]

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

/run-all-tests

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[ti-chi-bot]

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

Commit hash: 31bad92

[codecov]

Codecov Report

Merging #7911 (31bad92) into master (c1eabda) will increase coverage by 0.19%.
Report is 1 commits behind head on master.
The diff coverage is 50.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7911      +/-   ##
==========================================
+ Coverage   73.32%   73.51%   +0.19%     
==========================================
  Files         435      436       +1     
  Lines       48195    48341     +146     
==========================================
+ Hits        35337    35536     +199     
+ Misses       9784     9747      -37     
+ Partials     3074     3058      -16     

Flag	Coverage Δ
unittests	73.51% <50.00%> (+0.19%)	⬆️

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