client: update the leader even if the connection creation fails (#7443)

[ti-chi-bot]

This is an automated cherry-pick of #7443

What problem does this PR solve?

Issue Number: Close #7416

What is changed and how does it work?

update the leader even if the connection creation fails.
If connection is nil, consider it as networkErr

Check List

Tests

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

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:

• JmPotato

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[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

1. It must be approved by the approvers firstly.
2. AFTER it has been approved by approvers, please wait for the cherry-pick merging approval from triage owners.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b092996) 75.39% compared to head (cf0c8c0) 75.47%.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-6.1    #7473      +/-   ##
===============================================
+ Coverage        75.39%   75.47%   +0.08%     
===============================================
  Files              308      308              
  Lines            30591    30601      +10     
===============================================
+ Hits             23063    23097      +34     
+ Misses            5516     5497      -19     
+ Partials          2012     2007       -5     

Flag	Coverage Δ
unittests	75.47% <100.00%> (+0.08%)	⬆️

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[CabinfeverB]

This is a special case, not solved in the release branch