election: keepalive shouldn't exit early

[rleungx]

What problem does this PR solve?

Issue Number: Close #xxx

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

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]

Skipping CI for Draft Pull Request.
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[ti-chi-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.29%. Comparing base (5999d64) to head (b4bd7eb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8960      +/-   ##
==========================================
+ Coverage   76.22%   76.29%   +0.06%     
==========================================
  Files         464      464              
  Lines       70516    70515       -1     
==========================================
+ Hits        53754    53800      +46     
+ Misses      13407    13364      -43     
+ Partials     3355     3351       -4     

Flag	Coverage Δ
unittests	76.29% <ø> (+0.06%)	⬆️

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