manager: only sync label when label key exists

[ZiMengSheng]

Ⅰ. Describe what this PR does

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

V. Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in make test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.62%. Comparing base (5de95c3) to head (2555cab).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2070   +/-   ##
=======================================
  Coverage   68.62%   68.62%           
=======================================
  Files         430      430           
  Lines       39534    39536    +2     
=======================================
+ Hits        27130    27133    +3     
+ Misses      10061    10060    -1     
  Partials     2343     2343           

Flag	Coverage Δ
unittests	68.62% <100.00%> (+<0.01%)	⬆️

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[saintube]

/lgtm

[FillZpp]

/lgtm

[koordinator-bot]

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